Yes that might be the case.
At this stage I can still open the old FTM and open the unconverted trees, but not the tree which was converted to the 2017 format.
I'm a bit undecided now whether I bypass 2017 in favour of the new 2019 which is due out soon. The advantage of upgrading from 2014 being that I can sync and download up to date copies of my trees periodically. I could do that on RootsMagic, as mentioned in a previous post, if the Ancestry sync on that was working properly but at the moment it isn't.
In the past, I have tended to use RM and FTM as backups really, in addition to backing up to an external drive. After the amount of work put into research, I'd hate to lose any of it just because I didn't have good backups.
Anyway, I now know that if I use a newer version of FTM, the files can no longer be opened on the old - so a matter of ditching the old once I'm confident the new one has no unforeseen bugs.
Regards Eliza.