This is no small feat. I have been struggling with this all day and to the best of my knowledge, you are really stuck. It almost seems you would be better off creating a whole new project, and rename/id everything. Yes, workflows support versioning, and so do infopath forms but you do this at your own risk and there is little documentation on how to do this with any certainty. I have been struggling with this for some time now. InfoPath has the advantage of easy to create forms but really painful to deploy using a solution package. I couldn't get my new forms to publish correctly over an old one....everything seemed like it upgraded fine but I couldn't get the new form to show in my newly deployed workflow.
I think your safest bet would be to rename everything and create a new project, this way you will not have any issues if you ever decide to uninstall the previous workflow. Keeping them completely independant will be your safest option....let me know how it goes, I am curious to know. The topic is hard to Google...I know, I have tried.