Thanks, for me there were thousands of sources that should be deleted and and almost the same amount of citations to be changed, so I will wait with syncing a couple of days more so I don't see any horror stories before trying myself.
I do all my changes on-line on Ancestry, so even if I already have a backup of the FTM file I trust what's online more. And if a faulty sync makes my Ancestry and FTM trees the same - and both faulty - then a backup of the FTM file from before the sync is no cure-it-all. According to the blurb for FTM 2014 that TreeSync is "improved", and "lets you sync even more of your family tree info". Which indicates that the current TreeSync doesn't sync all family tree info, so my FTM file doesn't contain everything I entered in my Ancestry tree and an upload of that to a new Ancestry tree will miss some things.