diff --git a/Marlin/src/gcode/feature/advance/M900.cpp b/Marlin/src/gcode/feature/advance/M900.cpp
index fd500261b6..a8384108b5 100644
--- a/Marlin/src/gcode/feature/advance/M900.cpp
+++ b/Marlin/src/gcode/feature/advance/M900.cpp
@@ -29,11 +29,10 @@
 #include "../../../module/stepper.h"
 
 #if ENABLED(EXTRA_LIN_ADVANCE_K)
-  float saved_extruder_advance_K[EXTRUDERS];
+  float other_extruder_advance_K[EXTRUDERS];
   uint8_t lin_adv_slot = 0;
 #endif
 
-
 /**
  * M900: Get or Set Linear Advance K-factor
  *  T<tool>     Which tool to address
@@ -43,100 +42,107 @@
  */
 void GcodeSuite::M900() {
 
+  auto echo_value_oor = [] (const char ltr, const bool ten=true) {
+    SERIAL_CHAR('?'); SERIAL_CHAR(ltr);
+    SERIAL_ECHOPGM(" value out of range");
+    if (ten) SERIAL_ECHOPGM(" (0-10)");
+    SERIAL_ECHOLNPGM(".");
+  }
+
+  if (!parser.seen_any()) {
+
+    #if ENABLED(EXTRA_LIN_ADVANCE_K)
+
+      #if EXTRUDERS < 2
+        SERIAL_ECHOLNPAIR("Advance S", int(new_slot), " K", kref, "(S", int(!new_slot), " K", lref, ")");
+      #else
+        LOOP_L_N(i, EXTRUDERS) {
+          const bool slot = TEST(lin_adv_slot, i);
+          SERIAL_ECHOLNPAIR("Advance T", int(i), " S", int(slot), " K", planner.extruder_advance_K[i],
+                            "(S", int(!slot), " K", other_extruder_advance_K[i], ")");
+          SERIAL_EOL();
+        }
+      #endif
+
+    #else
+
+      SERIAL_ECHO_START();
+      #if EXTRUDERS < 2
+        SERIAL_ECHOLNPAIR("Advance K=", planner.extruder_advance_K[0]);
+      #else
+        SERIAL_ECHOPGM("Advance K");
+        LOOP_L_N(i, EXTRUDERS) {
+          SERIAL_CHAR(' ', '0' + i, ':');
+          SERIAL_ECHO(planner.extruder_advance_K[i]);
+        }
+        SERIAL_EOL();
+      #endif
+
+    #endif
+
+    return;
+  }
+
   #if EXTRUDERS < 2
     constexpr uint8_t tool_index = 0;
   #else
     const uint8_t tool_index = parser.intval('T', active_extruder);
     if (tool_index >= EXTRUDERS) {
-      SERIAL_ECHOLNPGM("?T value out of range.");
+      echo_value_oor('T', false);
       return;
     }
   #endif
 
+  float &kref = planner.extruder_advance_K[tool_index],
+        &lref = other_extruder_advance_K[tool_index];
+  const float oldK = kref, oldOther = lref;
+  float newK = oldK;
+
   #if ENABLED(EXTRA_LIN_ADVANCE_K)
 
-    bool ext_slot = TEST(lin_adv_slot, tool_index);
-
-    if (parser.seenval('S')) {
-      const bool slot = parser.value_bool();
-      if (ext_slot != slot) {
-        ext_slot = slot;
-        SET_BIT_TO(lin_adv_slot, tool_index, slot);
-        planner.synchronize();
-        const float temp = planner.extruder_advance_K[tool_index];
-        planner.extruder_advance_K[tool_index] = saved_extruder_advance_K[tool_index];
-        saved_extruder_advance_K[tool_index] = temp;
-      }
+    const bool old_slot = TEST(lin_adv_slot, tool_index), // The tool's current slot (0 or 1)
+               new_slot = parser.boolval('S', old_slot);  // The passed slot (default = current)
+
+    // If a new slot is being selected swap the current and
+    // saved K values. Do here so K/L will apply correctly.
+    if (new_slot != old_slot) {                       // Not the same slot?
+      SET_BIT_TO(lin_adv_slot, tool_index, new_slot); // Update the slot for the tool
+      newK = oldOther;                                // Get new K value from backup
+      lref = oldK;                                    // Save K to backup
     }
 
+    // Set the main K value. Apply if the main slot is active.
     if (parser.seenval('K')) {
       const float newK = parser.value_float();
-      if (WITHIN(newK, 0, 10)) {
-        if (ext_slot)
-          saved_extruder_advance_K[tool_index] = newK;
-        else {
-          planner.synchronize();
-          planner.extruder_advance_K[tool_index] = newK;
-        }
-      }
-      else
-        SERIAL_ECHOLNPGM("?K value out of range (0-10).");
+      if (!WITHIN(newK, 0, 10)) echo_value_oor('K');
+      else if (new_slot)        lref = newK;          // S1 Knn
+      else                      newK = newK;          // S0 Knn
     }
 
+    // Set the extra K value. Apply if the extra slot is active.
     if (parser.seenval('L')) {
       const float newL = parser.value_float();
-      if (WITHIN(newL, 0, 10)) {
-        if (!ext_slot)
-          saved_extruder_advance_K[tool_index] = newL;
-        else {
-          planner.synchronize();
-          planner.extruder_advance_K[tool_index] = newL;
-        }
-      }
-      else
-        SERIAL_ECHOLNPGM("?L value out of range (0-10).");
-    }
-
-    if (!parser.seen_any()) {
-      #if EXTRUDERS < 2
-        SERIAL_ECHOLNPAIR("Advance S", ext_slot, " K", planner.extruder_advance_K[0],
-                          "(Slot ", 1 - ext_slot, " K", saved_extruder_advance_K[0], ")");
-      #else
-        LOOP_L_N(i, EXTRUDERS) {
-          const int slot = (int)TEST(lin_adv_slot, i);
-          SERIAL_ECHOLNPAIR("Advance T", int(i), " S", slot, " K", planner.extruder_advance_K[i],
-                            "(Slot ", 1 - slot, " K", saved_extruder_advance_K[i], ")");
-          SERIAL_EOL();
-        }
-      #endif
+      if (!WITHIN(newL, 0, 10)) echo_value_oor('L');
+      else if (!new_slot)       lref = newL;          // S0 Lnn
+      else                      newK = newL;          // S1 Lnn
     }
 
   #else
 
     if (parser.seenval('K')) {
       const float newK = parser.value_float();
-      if (WITHIN(newK, 0, 10)) {
-        planner.synchronize();
-        planner.extruder_advance_K[tool_index] = newK;
-      }
+      if (WITHIN(newK, 0, 10))
+        newK = newK;
       else
-        SERIAL_ECHOLNPGM("?K value out of range (0-10).");
-    }
-    else {
-      SERIAL_ECHO_START();
-      #if EXTRUDERS < 2
-        SERIAL_ECHOLNPAIR("Advance K=", planner.extruder_advance_K[0]);
-      #else
-        SERIAL_ECHOPGM("Advance K");
-        LOOP_L_N(i, EXTRUDERS) {
-          SERIAL_CHAR(' '); SERIAL_ECHO(int(i));
-          SERIAL_CHAR('='); SERIAL_ECHO(planner.extruder_advance_K[i]);
-        }
-        SERIAL_EOL();
-      #endif
+        echo_value_oor('K');
     }
 
   #endif
+
+  if (newK != oldK) {
+    planner.synchronize();
+    kref = newK;
+  }
 }
 
 #endif // LIN_ADVANCE
diff --git a/Marlin/src/module/configuration_store.cpp b/Marlin/src/module/configuration_store.cpp
index 346fd64de3..77fc18bb53 100644
--- a/Marlin/src/module/configuration_store.cpp
+++ b/Marlin/src/module/configuration_store.cpp
@@ -101,7 +101,7 @@
 #endif
 
 #if ENABLED(EXTRA_LIN_ADVANCE_K)
-  extern float saved_extruder_advance_K[EXTRUDERS];
+  extern float other_extruder_advance_K[EXTRUDERS];
 #endif
 
 #if EXTRUDERS > 1
@@ -2717,7 +2717,7 @@ void MarlinSettings::reset() {
     LOOP_L_N(i, EXTRUDERS) {
       planner.extruder_advance_K[i] = LIN_ADVANCE_K;
       #if ENABLED(EXTRA_LIN_ADVANCE_K)
-        saved_extruder_advance_K[i] = LIN_ADVANCE_K;
+        other_extruder_advance_K[i] = LIN_ADVANCE_K;
       #endif
     }
   #endif