I happened to vaguely remember some details of migration from Version 16. Perhaps others having more recent experiences might have more details.
I think you will be pleased with the new facility for standardizing place names. It can also be used as a county finder, but be careful. There could be dozen townships and multiples cities and towns named Washington or Jefferson or Fremont, for example. The old fashioned county finders on the web are better for that purpose; or even try Wikipedia - which has disambiguation pages for multiple names of places in a state.
From what I remember, however, you will have a major project in front of you in fixing sources. I don't recall the specifics of the problems, but after you import into FTM, check your sources to see what, if anything, is mixed up. You might setup a test file to enter a person and facts and sources to see how the new format works. See how the Sources Workspace is setup and how it works. Then develop your conversion strategy for sources.