I have two iPhones that are on XM and one that is not (unlocked iPhone 8, never used on XM)
Can I just swap SIM's on these phones if I want to change which one has which number? (the phones have different specs) they should all be supported radio wise.
If I can't just swap SIM's what do you have to do to move numbers between them?
For the most part swapping SIM cards between your phones should work fine. The phone number follows the SIM card.
Take a look at the following thread and the thrid post, ComcastChrisL explains the possible issues with swapping SIM Cards.