Is it really necessary to return my license and then reinstall everything?
You only have to return your licence if you are going to reformat your hard disk/reinstall Windows/transfer Trados to a new machine.
However, if you want to be quite certain, returning a licence can't do any harm. All you do is return the licence, uninstall and reinstall the program and activate the licence again.
And would I risk loosing all contact with my current TMs and MT databases unless I first find them all and save them?
Well, it depends where you have saved them. I save mine in a completely different folder and so they are never touched by any reinstallation procedures.