I have two main categories; i want to move all the runs from one into the other. I could edit each of the runs individually, but is there a way that I can just move them all in one go? They have the same subcategories, too.

As far as I know, you can’t do this.

For large amounts of runs, sometimes the admins will step in and help, but for a smaller number of runs, it’s probably faster for you to do it yourself by manually moving them.

It's around 100 runs, I believe.

Trying to merge https://www.speedrun.com/cuphead/full_game#All_Bosses_-_Glitchless into https://www.speedrun.com/cuphead/full_game#All_Bosses , FWIW.

I'll start doing it manually, thanks for the help anyways

Admins can do mass migration of runs, but you can probably get it done before they get around to it :)