When we last switched it was from one vendor to another (Pro/Whatever-they-call-it to Solidworks/Smarteam) so we went in with the mindset of not keeping files in new PDM from old CAD so re remodeled as required. I know this isn't much help but I feel that translation isn't really the direction to go in.
- Do you have to use a 3rd Party tool/software? - This all depends on what tools the destination vendor has. I'm able to migrate data from many different PDM systems with out of the box tools in SolidWorks PDM. We can usually write migration tools via XML for the crazy stuff assuming there's data to access. Custom anything will add $$ and time to any project.
- Do you lose editability of the files? Assembly mates, etc.? - Most imports bring dumb solids although we did have a Pro/E importer that could re-create features and mates at one point. I would test this outside of the PDM migration.
- Do you lose past versions/revisions/history of the files? - See my first answer. Lots of tools can get revision history if it's there to begin with and accessible. You've kept your question very generic so the answers here will also be very high level and generic.
- Do you lose the custom properties/data card info, or anything like that? - Usually no especially if they are also in the PDM database. If they are only in the files themselves, then you stand to lose all of that data if the importers can't capture it.
- Also, do the drawings come over cleanly or somewhat cleanly? - I've never seen drawings come over (I may had limited exposure though). Parts and assemblies import great, but drawings usually are left behind. Besides, don't you want to forget about drawings and use MBD instead
If you can re-phrase your question with more specifics, there are plenty of members who can give more specific replies. I will say that migrating data is never easy and data loss can sometimes not be avoided.
I appreciate all the info Steve.