Unable to Delete Project Checklist Types

I am currently trying to delete some Project Checklist Types that are legacy items, but I am unable to do so through Checklist Type Maintenance. I believe this is due to the fact that the types are still linked to the older projects that exist and therefore are still in use. Has anyone gone through this before?

I assume the best way to get around this would to be to delete all the old project checklist items in the projects, or migrate them to a general “legacy” checklist item. Which is the better option, and how would you go about accomplishing the chosen task?