I did the same thing. Was running FTM 2005 (on Windows 7). I was getting concerned that it may stop working so I bought FTM 2019. (I did a live chat with Mackiev and got a reasonable upgrade price). On opening FTM 2019 you point it at the folder where your original files sit (*.FTW and *.FTB files from FTM 2005). It then builds a new file from that and saves it where you ask. There is no use of *.GED files in this process. The FTM 2019 format is obviously very different, so you cannot go back to FTM 2005 to read the new file. You can choose to run both FTMs if you like, but there is no obvious way to keep the two files in sync.
I'm happy with FTM 2019 but is much slower than old version because it keeps all windows open and updated at once - Tree view, name list, relationships... Also, on the PC, the fonts used are rather small. There are some options to adjust that.
I do sync with Ancestry, but I don't find much use for the integration of Ancestry and FTM. I prefer to deal with "hints" manually by doing my own research.
Like any software, there's a learning curve to deal with the new program.