Can one replace a tubeless tire with a tube? I have a tubeless tire on a mountain bicycle which is old and wasn't taken care of, and I want to replace it with something more maintainable, as (according to the YouTube videos I've seen) tubeless tires are harder to change.

Would I buy a tube and new tire and replace the old tire with it, or are the rims different?
All tubeless compatible rims can run tubes - that is how you deal with a puncture in on the track. 

As far as tires, you can go with any tire that is the correct size for the rim. If you get a tubeless-ready tire, its possible to run a tube and then go back to tubeless using a sealant like slime or Stan's, and they are much lighter than a tubed tire.