Unable to "backport" litematicas, but I need to, any help? Willing to do anything (even mess with coding)

[Ianoneshot]

Good morning, afternoon, or night,

Me and my friend have been making a souls-like map for about a year now. I made working bosses in a 1.16.5 version of the map (as well as stuff like working "bonfire" mechanics and rolling system with animations). However, he is the builder, and accidentally updated the world to a 1.19 version over the years. In other words, he has been placidly updating it not realizing backports of full worlds are not possible.
So I devised a plan, "I'll use 1.18 litematica to make schematics of the already-existing content he made, and paste them into the 1.16.5 version!" After all, he did not use any block from anything that stemmed from 1.17-1.19 .
However, I tried this tactic, and it says "unsupported future version (6)" when I try to load the litematica schematics into the 1.16.5 version of the world.
We worked so hard on this adventure map, it even has custom music and resources! We do not want it to go all to waste now because of a mistake. Is there a way to bypass the "unsupported future version" of the schematic, even if the schematic might not technically be "complete"? We are willing to take anything at this point, and we are really desperate for some solution.

Thank you so much for reading, any help will be greatly appreciated.

Sincerely,
Ianoneshot and UndefinedTactics

[warjort]

The suggested answer has already posted on your litematics issue
maruohon/litematica#543

i.e. Use an nbt editor to change the version number in a copy of your file.
Of course, it might still fail to load in 1.16.5 if you have used blocks from 1.17+?
Then you will need to edit those blocks to substitute a different block identifier.