TMG can read the UFT database directly from the original UFT file or from any "FULL" UFT backup file. It does not change the UFT file in any way but reads the data and creates a TMG database that you can check against the UFT database.
If you do not like the options you choose (or the default options that TMG chooses for you) you may do this process over again, with different import options, as many times as you wish until you get the results you like. This sounds complicated but it really is not and the default options are normally the ones you will like.
If you do not like any of the results, Wholly Genes Software has a 90 day full refund policy so you do not have to keep TMG if you do not like it.