If the change you've made did not modify the type of the function, then updating all usages of that function is as simple as running "patch".
If you run "todo" it will show you everything that needs to be updated (thought it doesn't force you to update anything).
If you change the type of the function, you'll need to update all usages one by one, or choose to leave some usages with the old function (which may be desirable in some circumstances, e.g. where the bug you fix is never reached from that caller and you want to be maximally backwards-compatible)... UCM will show you everywhere you need to do that... you can get all references to a function before you modify it as well by running "dependents my-fun".
If you run "todo" it will show you everything that needs to be updated (thought it doesn't force you to update anything).
If you change the type of the function, you'll need to update all usages one by one, or choose to leave some usages with the old function (which may be desirable in some circumstances, e.g. where the bug you fix is never reached from that caller and you want to be maximally backwards-compatible)... UCM will show you everywhere you need to do that... you can get all references to a function before you modify it as well by running "dependents my-fun".