Does it make a difference if the version 17.2 is modded? My version of 18.2 did not find the saves and when I moved the save folder into 18.2 they don't work.
Is it possible to just overwrite 17.2 with 18.2?
Why would you want to overwrite one version with another? Just delete the old version. At least on PCs (Windows and Linux), but probably also on Macs, the save files are stored in two locations to make this work without losing your saves. If for some reason you need to manually preserve the save files (e.g. new PC and not played the game on it yet), just move the subfolder
game/saves to some other location, then delete 17.2, unpack 18.2, and then move subfolder game/saves back over the new one.
Overwriting often works without issues, but it can easily lead to severe problems. Not just if you originally had a mod that breaks things if you don't update it. A lot of minor ways in which a developer might change the organization of the game can break if you do this.