Handle this at a lower level.

service/fit.py

@@ -726,8 +726,14 @@ class Fit(object):
         else:
             fit.drones.remove(d1)
 
-        d2.amount += d1.amount
-        d2.amountActive += d1.amountActive if d1.amountActive > 0 else -d2.amountActive
+        d2.amount += getattr(d1, "amount", 0)
+        d2.amountActive += getattr(d1, "amountActive", 0)
+
+        # If we have less than the total number of drones active, make them all active. Fixes #728
+        # This could be removed if we ever add an enhancement to make drone stacks partially active.
+        if getattr(d2, "amount", 0) > getattr(d2, "amountActive", 0):
+            d2.amountActive = getattr(d2, "amount", 0)
+
         eos.db.commit()
         self.recalc(fit)
         return True