If I have a long running Workflow (that lasts for months....) and if I need to for example add a new state or change it. (because of a bug fix or new feature).
What happens to existing workflows? will they be lost? or do I need to some versioning? or will it accept the new state an go on?
Provided the workflow can rehydrate correctly I'd think that it will pick up the new assemblies.
Changes to the workflow will not affect workflows already executing. The entire workflow is serialized to your persistence store in its current state.
If you want to modify an existing workflow, you need to actually go through a bunch of hoops to make that happen.