diff --git a/GAME.md b/GAME.md
index ce8eecd..ac95fe9 100644
--- a/GAME.md
+++ b/GAME.md
@@ -130,6 +130,7 @@ The app models this with the New item comparison panel:
 - Enter the new roll's base stat and effects.
 - The app shows DPS and Regen change separately.
 - If `Pet` is selected, the app chooses the current worst pet by piece score as the replacement target.
+- Swapping in the new roll stores the replaced piece and prior New item fields so the user can swap back.
 
 ## Profiles
 
diff --git a/README.md b/README.md
index 78aa1c2..7ec6ad3 100644
--- a/README.md
+++ b/README.md
@@ -26,6 +26,7 @@ For the game rules, stat caps, formulas, assumptions, and known limitations used
    - choose what it would replace,
    - enter the new roll's value and effects,
    - compare DPS and Regen changes before swapping.
+   - use **Swap in new item** to apply it, with **Swap back** available if you want to undo.
 7. Use **Best sacrifice candidates** as a quick shortlist of low-value pieces.
 
 When comparing a new pet, choose `Pet`; the app automatically compares against your currently weakest pet.
diff --git a/app.js b/app.js
index 321d068..0221a7a 100644
--- a/app.js
+++ b/app.js
@@ -71,6 +71,7 @@ function createDefaultNewItem() {
     baseDamage: 0,
     baseHealth: 0,
     effects: [{ type: "", value: 1 }, { type: "", value: 1 }],
+    lastSwap: null,
   };
 }
 
@@ -93,13 +94,14 @@ function createDefaultPiece(slot, index) {
 function enforceFixedPieceFields(piece, index) {
   const fixed = FIXED_ITEMS[index];
   if (!fixed || piece.type !== "item") return piece;
+  const baseValue = Number(piece.baseValue || (fixed.baseType === "damage" ? piece.baseDamage : piece.baseHealth) || fixed.baseValue);
   return {
     ...piece,
     name: fixed.name,
     baseType: fixed.baseType,
-    baseValue: fixed.baseValue,
-    baseDamage: fixed.baseType === "damage" ? fixed.baseValue : 0,
-    baseHealth: fixed.baseType === "health" ? fixed.baseValue : 0,
+    baseValue,
+    baseDamage: fixed.baseType === "damage" ? baseValue : 0,
+    baseHealth: fixed.baseType === "health" ? baseValue : 0,
     fixedBase: true,
   };
 }
@@ -147,6 +149,7 @@ function normalizeNewItem(candidate) {
     baseDamage: Math.max(0, Number(candidate?.baseDamage || 0)),
     baseHealth: Math.max(0, Number(candidate?.baseHealth || 0)),
     effects,
+    lastSwap: candidate?.lastSwap || null,
   };
 }
 
@@ -296,6 +299,7 @@ function renderNewItemComparison() {
       </label>
       <label>Percent
         <input data-new-item-effect-value="${effectIndex}" type="number" min="1" step="0.1" value="${effect.value}" />
+        <span class="cap-hint"></span>
       </label>`;
     row.querySelector("select").value = effect.type;
     effects.appendChild(row);
@@ -343,6 +347,29 @@ function renderReplacementComparison(totals, currentDps, currentRegen, pieceScor
   $("#replacementDpsDelta").className = deltaClass(dpsDelta);
   $("#replacementRegenDelta").textContent = formatDelta(regenDelta, "%");
   $("#replacementRegenDelta").className = deltaClass(regenDelta);
+  updateNewItemImpactHints(replacementTotals, replacementDps);
+  updateSwapControls();
+}
+
+function updateNewItemImpactHints(replacementTotals, replacementDps) {
+  document.querySelectorAll("#newItemEffects .effect-row").forEach((row, effectIndex) => {
+    const effect = state.newItem.effects[effectIndex];
+    const hint = row.querySelector(".cap-hint");
+    if (!effect?.type) {
+      hint.textContent = "Ignored";
+      return;
+    }
+    const current = clampEffectValue(effect.type, Number(effect.value));
+    const withoutThis = calculateDps(cloneTotalsWithEffectDelta(replacementTotals, effect.type, -current), state.combatStyle);
+    const currentGain = percentChange(withoutThis, replacementDps);
+    hint.textContent = `Actual: ${fmt(currentGain)}% DPS`;
+  });
+}
+
+function updateSwapControls() {
+  const hasSwap = Boolean(state.newItem.lastSwap);
+  $("#swapBackNewItem").disabled = !hasSwap;
+  $("#swapStatus").textContent = hasSwap ? `Can swap back ${state.newItem.lastSwap.piece?.name || "previous item"}.` : "";
 }
 
 function calculateTotals() {
@@ -404,6 +431,68 @@ function createNewItemCandidate(target) {
   };
 }
 
+function createPieceFromCandidate(existingPiece, candidate) {
+  const baseDamage = Number(candidate.baseDamage || 0);
+  const baseHealth = Number(candidate.baseHealth || 0);
+  return enforceFixedPieceFields({
+    ...existingPiece,
+    baseDamage,
+    baseHealth,
+    baseType: existingPiece.fixedBase ? existingPiece.baseType : baseHealth > baseDamage ? "health" : "damage",
+    baseValue: existingPiece.fixedBase
+      ? (existingPiece.baseType === "health" ? baseHealth : baseDamage)
+      : Math.max(baseDamage, baseHealth),
+    effects: candidate.effects.map((effect) => ({ ...effect })),
+  }, state.pieces.indexOf(existingPiece));
+}
+
+function clonePiece(piece) {
+  return {
+    ...piece,
+    effects: (piece.effects || []).map((effect) => ({ ...effect })),
+  };
+}
+
+function cloneNewItem(newItem) {
+  return {
+    ...newItem,
+    effects: newItem.effects.map((effect) => ({ ...effect })),
+    lastSwap: null,
+  };
+}
+
+function swapInNewItem() {
+  const profile = PROFILES[state.profile];
+  const totals = calculateTotals();
+  const pieceScores = state.pieces.map((piece) => scorePiece(piece, profile, totals));
+  const targetPiece = getReplacementTargetPiece(state.newItem.target, pieceScores);
+  if (!targetPiece) return;
+  const index = state.pieces.indexOf(targetPiece);
+  const candidate = createNewItemCandidate(state.newItem.target);
+  const previousPiece = clonePiece(targetPiece);
+  const previousNewItem = cloneNewItem(state.newItem);
+  state.pieces[index] = createPieceFromCandidate(targetPiece, candidate);
+  state.newItem = {
+    ...previousNewItem,
+    lastSwap: {
+      index,
+      target: previousNewItem.target,
+      piece: previousPiece,
+      newItem: previousNewItem,
+      swappedAt: new Date().toISOString(),
+    },
+  };
+  render();
+}
+
+function swapBackNewItem() {
+  const lastSwap = state.newItem.lastSwap;
+  if (!lastSwap || !state.pieces[lastSwap.index]) return;
+  state.pieces[lastSwap.index] = clonePiece(lastSwap.piece);
+  state.newItem = cloneNewItem(lastSwap.newItem);
+  render();
+}
+
 function formatDelta(value, suffix = "") {
   const sign = value > 0 ? "+" : "";
   return `${sign}${fmt(value)}${suffix}`;
@@ -747,6 +836,8 @@ function bindEvents() {
     renderNewItemComparison();
     updateAnalysis();
   });
+  $("#swapNewItem").addEventListener("click", swapInNewItem);
+  $("#swapBackNewItem").addEventListener("click", swapBackNewItem);
   $(".new-item-card").addEventListener("input", handleNewItemInput);
   $(".new-item-card").addEventListener("change", handleNewItemInput);
 }
diff --git a/index.html b/index.html
index c7f8d48..f5fecd0 100644
--- a/index.html
+++ b/index.html
@@ -104,6 +104,11 @@
           <div><span>DPS change</span><strong id="replacementDpsDelta">0</strong></div>
           <div><span>Regen change</span><strong id="replacementRegenDelta">0</strong></div>
         </div>
+        <div class="swap-actions">
+          <button id="swapNewItem" type="button">Swap in new item</button>
+          <button id="swapBackNewItem" type="button" class="secondary" disabled>Swap back</button>
+          <span id="swapStatus" class="status"></span>
+        </div>
       </section>
 
       <section>
diff --git a/styles.css b/styles.css
index 02d9404..4bcd6cc 100644
--- a/styles.css
+++ b/styles.css
@@ -255,6 +255,13 @@ h3 { margin-bottom: 0; }
 .delta-positive { color: #86efac; }
 .delta-neutral { color: var(--muted); }
 .delta-negative { color: #f87171; }
+.swap-actions {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.75rem;
+  align-items: center;
+}
+.swap-actions .status { margin: 0; }
 
 @media (max-width: 900px) {
   .hero, .results { grid-template-columns: 1fr; }