Thanks. It wouldn't let me select areas for repop while I also had selected areas for pruning without overlapping them and having my entire area/base selected for repop, so I'll go ahead and do the pruning first, then do the repop after and only select the borders, as you said. Hopefully that will all work.
So... possibly stupid/redundant question. I checked out that link you posted explaining why you only need to repop those two boundaries, and I'm not sure if I understand. Would it be bad to select all four boundaries for repop? I just don't want to only have ores/mobs/etc. ONLY in areas I generate to the south and east- I'd like to have everything generate normally everywhere.
The vertical walls are the second picture. I do know the original seed! Is there a way to turn it into it? To sort of "paste" the part of my world I want to keep into the original seed? The part of my world I want to keep was generated in that original seed in the first place.
So.... It didn't work. xD The world didn't regenerate according to the original seed at all, and it didn't solve the abrupt-transition-vertical-wall problem... It just created those vertical walls in a box around the area I kept in the pruning process, as shown in the image. What could be going wrong?
When I first picked out the seed for my world, I was thrilled, because all of the rare biomes were sort of clustered nearish spawn. But then I had my world corrupted a couple of times (no worries- I learned my lesson and have since been backing it up.) After the world corruption I ended up having chunks transformed, so you see wide swaths of land where it is vertically cut off (vertically flat mountains, etc). Beyond those vertical line areas everything was transformed, with different terrain having been randomly generated. Because of this, I lost my mesa. And there is no mesa at all, anywhere near me.
I created a new world with the seed to see if the world generation is the same in 1.12, and it is- the original pre-corruption world is there, mesa and all. But of course, I didn't just start over, because I've already built a ton in my corrupted world.
Then I watched Xisuma's recent video where he clips parts off of the hermitcraft server using MC edit. So here is my question- would snipping areas in my world using MC edit reset the chunks, and if so, would they then generate the same as they did originally? Would they generate according to the regular seed? Would I get rid of those long stretches of vertical flatness and get my mesa back? Or would the terrain randomly generate in new ways after the snipping reset?