Weird. The replay for Tension 39 should still work, as the only thing that changed is the save requirement. A few versions ago, I managed to save 1 extra on Tension 39. With the newer version, Icho changed the save requirement so that it is now exactly the amount I saved, so this should not had caused the replay to break. Yes, you will see a yellow check mark for Tension 39, but it doesn't always mean the old replay solution will break. In that case, you simply just need to run a mass replay check and it should had turned to a green checkmark no problem. And yes, several War replays broke because Icho made changes to those levels that cause my solutions to not work anymore, which means I now need to resolve them again. Several assignments in them should still be the same, but some of them will need to be changed so that it's a working solution.
Regarding Neutrality 40, you'll going to have to wait a while for that one, when I send replays for all levels, including Genocide and the Bonus ranks. It's definitely a tough one, and I remember that one taking me a while to solve. The difficulty is in large part due to how the map is really big and also so many things are happening at once that it's not obvious what/where the skills need to be used. In Icho's explanation of why it's a difficult level, the level has extremely high entropy: so many things happening and skill placements are somewhat non-obvious in that they can't automatically be placed right away.
Ah, thanks for being honest about at least attempting some of the United levels. They're quite satisfying when you finally figure the solutions out on your own. And don't worry, it's not like I myself never ever get stumped. Sometimes, it takes me hours or even days/weeks depending on the level, but if I keep at it, eventually I'll get it.