diff --git a/CHANGELOG.md b/CHANGELOG.md
index 554039050a..a5cb664c2c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Features
 
+-   Added functionality to initialize a model using the solution from another model ([#1278](https://github.com/pybamm-team/PyBaMM/pull/1278))
 -   Added submodels for active material ([#1262](https://github.com/pybamm-team/PyBaMM/pull/1262))
 -   Added composite surface form electrolyte models: `CompositeDifferential` and `CompositeAlgebraic` ([#1207](https://github.com/pybamm-team/PyBaMM/issues/1207))
 
@@ -11,6 +12,7 @@
 
 ## Bug fixes
 
+-   Fixed `Simulation` and `model.new_copy` to fix a bug where changes to the model were overwritten ([#1278](https://github.com/pybamm-team/PyBaMM/pull/1278))
 ## Breaking changes
 
 -   Operations such as `1*x` and `0+x` now directly return `x`. This can be bypassed by explicitly creating the binary operators, e.g. `pybamm.Multiplication(1, x)` ([#1252](https://github.com/pybamm-team/PyBaMM/pull/1252))
diff --git a/pybamm/expression_tree/operations/replace_symbols.py b/pybamm/expression_tree/operations/replace_symbols.py
index 1b54addbd3..34f60aa573 100644
--- a/pybamm/expression_tree/operations/replace_symbols.py
+++ b/pybamm/expression_tree/operations/replace_symbols.py
@@ -138,7 +138,7 @@ def process_boundary_conditions(self, model):
                     if err.args[0] in side:
                         pass
                     # do raise error otherwise (e.g. can't process symbol)
-                    else:
+                    else:  # pragma: no cover
                         raise KeyError(err)
 
         return new_boundary_conditions
@@ -170,11 +170,6 @@ def process_symbol(self, symbol):
 
     def _process_symbol(self, symbol):
         """ See :meth:`Simplification.process_symbol()`. """
-        if not any(
-            x.id in self._symbol_replacement_map_ids.keys() for x in symbol.pre_order()
-        ):
-            return symbol.new_copy()
-
         if symbol.id in self._symbol_replacement_map_ids.keys():
             return self._symbol_replacement_map_ids[symbol.id]
 
@@ -202,10 +197,8 @@ def _process_symbol(self, symbol):
             return symbol._concatenation_new_copy(new_children)
 
         else:
-            if not any(
-                x.id in self._symbol_replacement_map_ids.keys()
-                for x in symbol.pre_order()
-            ):
-                return symbol.new_copy()
-            else:
-                raise NotImplementedError
+            # Only other option is that the symbol is a leaf (doesn't have children)
+            # In this case, since we have already ruled out that the symbol is one of
+            # the symbols that needs to be replaced, we can just return a new copy of
+            # the symbol
+            return symbol.new_copy()