Hi Withers,
By 'My existing workflows' you mean workflows that were in the middle of running? I think you may have an issue that is not easy to fix.
You may have to manually specify the new user in any view and workflow. As for the running workflows I think you are out of luck there too. I ran into a similar issue last year and did not find an elegant solution. The later rollup did alleviate the issue where now you can get workflow to automatic map if you re-map users, but i found no easy fix for the workflows that were in the middle of running.
Just in case - do you have update rollup 7 installed ?
Alex Fagundes -
www.PowerObjects.com