The process started with one process definition version continue with this version until it finish..
If you deploy a new version of one process the version number is incremented and a new definition is inserted in the database. So, all the new process instances will be with the new definition, and the process that are already running continue with the old definition.
Hope it helps.
That helps but what if we have to modify the workflow even for in-process workflows? Is that difficult? Has anyone tried? Probably just modify the database tables, but could be involved.
I think you could do this, but you be aware of possible inconsistences between the process states.