MCA Selector is an "external tool to export or delete selected chunks and regions from a world save of Minecraft Java Edition" github.com/Querz/mcaselector
My lesson is: Do not upgrade, then turn around and downgrade. Loss of all stuff in entire world.
I updated my experimental world (fullbright resource pack from vanilla tweaks) to 1.20.6 and found fullbright did not work. So, I restarted in 1.20.4 the original world, but lost all of my inventory, armor, and anything in a chest, which was quite substantial. Since I didn't want to go through the hard work of acquiring these things, I used MCA Selector to copy and past from a backup world, because I did want to use 1.20.6 even if it didn't have fullbright resource pack.
So in the backup world from 1.20.4, I took off my armor and tools from my inventory and put in a blue shulker box. I placed the box in a one block space in front of buildings to see it easily. Then I logged out.
In MCA selector, I have both worlds pulled up and put side by side. I copied the first world small shulker box with my armor, weapons and tools, moved to the second world and pasted it within the same block.
I hope you can use this tool to save your worlds and inventory that got corrupted somehow.