It was pretty painless for me. Mileage might vary I suppose. I had more troubles with going from win7 to win10 since avspmod stopped working for me. Somebody has patched it since then. Definitely backup your plugins folder to as many USB sticks as you can find. Worst case scenario, you can roll back to 2.5.8 if all else fails.

I have a lot of external filters loaded and there were maybe 2 that didn’t work right out of the cage with 2.6. It’s usually just a matter of finding the latest version on the wiki and overwriting it in the plugins folder.