I did some farting around yesterday just to see if that .dat thing might mean structures and whatnot would be placed differently between versions, and the results were: not really. Out of 5 villages, 2 temples and 1 monument found in a random 1.12 seed, all but one village spawned at the same place in 1.13, with the other village just not there at all.
It also looks like the chest data for villages, temples, et al are stored in the level.dat file, not in the associated villages.dat etc. So yeah, chests would be empty or nonexistent if we just wiped with the same seed. So, new seed I guess?
Bummer, but not that huge of a deal.
I was meaning to move the 'Map Wall' away from downtown anyways as I suspect all of those item frames might be impacting performance, so if those maps go boink no biggie. I will build again.