I read more help files after writing my reply, and think I know what will happen.
The ancestry.com tree that I have will stay the same, which is what I want. After I break the link to my current FTMMac tree, and begin the download process, I will choose that ancestry.com tree [my only one] but can give the FTM version a slightly different name. So I end up with 1 ancestry.com tree, the one I want to keep, and 2 FTM trees, and I can delete the corrupted one that I wanted to keep just in case later.
I DO have FTM backups. Should I therefore just let the download over-write the same name tree [assuming that is what happens]? It will overwrite my current FTM file and Media folder, but I have backups from just before the sync problem started.
Sorry to be a pest, but I don't want to mess this up.