Reset nether but preserve above ceiling?

I'm running a Minecraft server (version 1.15.2), and I plan to reset the nether for the 1.16 update.

I would like to keep anything that's been built above the nether ceiling. Obviously, resetting the nether by removing the region files will destroy everything. Is there a way to selectively copy the above-ceiling content back from the original region files to the regenerated ones? I'm thinking a workflow something like this:

1. Update to 1.16.
2. Delete the old nether region files.
3. Force-generate the nether regions within a certain area.
4. Copy the data from the old region files to the new, only for Y>128.

What I'm missing is a tool to accomplish step (4). Is there such a tool?

Solution 1:

You could load the chunks of a new nether dimension with the same seed and load it into NBTExplorer and open all of the old nether dimension's chunks too.

Then you could copy the highest sections of the old chunks into the new nether dimension.

I know this is very tedious and complicated but that's all I can think of - you will have to do this for every chunk and manually copy the files to and from the server.

Another option is manually coding your own program to do this, but that's a little hard. I will update this question if I manage to make a program that does that.