Buddy of mine decided he did not like the the gun after 50 rounds mixed buckshot and slugs, gave it to me. I have been looking for a while for a basic starter shotgun and now I have one and am very grateful. That being said I would like to put a 24 or 26 inch chokeable barrel on it as well for clays and birds, and have the 20 inch barrel easily swappable and the home defense barrel. Everything I am reading on the web says the 500 and 590 and even the maverick line can all share barrels. But then I found a couple of boards that say it cannot be done without changing the mag tube if it is the 8 round mag tube. So now I am concerned it cannot be done. I really like the 8+1 capacity but 20 inches and no chokes seems short for birds and skeet or clays. Plus it seems to be a pain to change the mag tube and barrel versus just swapping a barrel.

Tldr version is it possible to put a 24 or 26 inch barrel on a 590 with an 8 round mag tube in place of the 20 inch barrel without having to change the mag tube?

I've got the 500 with the long mag tube and I don't thin you can. You'll need to buy both, but I'm not an expert.

Me either, worst comes to worst I follow the n+1 rule and get a second one with the money I planned to use anyway to get a first one.
More internet searching is turning up more of what you said, apparently the 590 with the 20in barrel and 8 round mag tube have the connection point in a different spot than if it were the 500 or maverick with the shorter tubes. If it had been the 18.5in barrel and shorter mag tube then the barrels are interchangeable across the three models.
Which leads to a new question, how vital are chokes and longer barrel versus the extra 3 rounds?