Updating active Business Processes

I have a BPL Business process which always processes messages - mainly it waits for responses since downstream systems are seriously async.

I need to update this process.

Wwould the existing instances of this Business Process complete correctly after the update?

My BPL changes would result in:

1. New S method in the Thread class
2. New OnResponse method in the context class

Both changes are in the middle of the process, so there are S/OnResponse methods before and after my change.

What if it's only a new S method? What if it's only a new OnResponse method?