Maybe you could use a utility to fill in the sub-bedrock gap with bedrock for all the old chunks. I'm not familiar with modern map editors, but in the past, I would have used mcedit or pymclevel to do that.
Thanks guys. I guess I am panicking prematurely. The release hasn't even come out yet!
My devs informed me they plan on expanding the map to 30k x 30k when 1.17 is released, so my brain exploded with all the issues it could create!
This is going to be a massive undertaking if mojang doesnt have a migration path for server owners to implement.
Will just wait and see once it's released and plan from there.
Do any of you have a "Server Owner" Discord group or owner specific Forum you know of that I could join to keep up with others and their ideas?
Message me if its not allowed to be posted in here plz.