I have a modded Minecraft map that myself and a number of others have been buildings and developing for some time now. Recently, we finally started considering moving onto a more recent build of Minecraft on SpongeForge which was a viable upgrade from the Cauldron 1.7.10 in which we could keep all the custom blocks and continue to use plugins aside them, the former being the significantly more important part, whilst enjoying all the benefits of upgrading such new features for most mods and better general stability.
So after some discussion, I got to work as owner. I checked compatibility with the mods, to which all the important ones seemed to be compatible with the recent builds and spent a few days sorting out SpongeForge with all the mods being tested alongside a variety of plugins on a local-hosted server test world.
...And then I attempted to port the actual world.
I downloaded the map from the host to put it in an give it a test on the 1.12.2 modded server but there was one problem. The log seems to say that the save predates 1.7.10 despite having run on the latest 1.7.10 Cauldron build for months.
I've been left quite confused and unsure of what to do here. I tried loading it up in 1.7.10 single player but it loads just fine and re-downloading the map again, but both yield the same message. I can't exactly try to load it up in 1.8 as a number of important mods do not support that Minecraft version meaning that a lot of things will be lost/damaged. Finally, just restarting/making a new world isn't really much of an option considering the time investment so far.
So then, for those who have stuck around the end, do you have any suggestions? Any ideas on how to fix this would be appreciated.
Server Log for Specifics - https://pastebin.com/t3LsQdYd