This behavior might have changed in the newer versions, but I think I have a way that might work.
If you look for a place where you made an assignment to a, such as:
a = 2;
You can select the variable name and start typing a new name. You should see a tooltip pop up that says you can rename all uses of this variable by hitting Shift+Enter.
The problem with this is that AFAIK, you can't enter a dot indexing expression this way. The workaround would be to use a unique temporary variable name. Say you use this renaming feature to rename all uses of 'a' to:
someuniquename1234 = 2;
Now you can do a simple search and replace on all instances of the temporary unique name.
