Decided to post this since a quick google didn't return anything so I'm not sure this is widely known, just a bug, or just somewhat obscure. Light gameplay spoilers for chapter 7 ahead.

The method to unequipping Dromarch from Nia, Pandoria from Zeke or Brighid from Morag is quite simple; have said blade in the second or third slot of its driver, then have Rex equip it. Afterwards, Rex can unequip it or fight with it, and the slot of its original driver will be free for any other blade to use.

What is this useful for? Personally, I've used it to fulfill field skill requirements (say, you need Thunder Mastery level 6 but Nia has all your Thunder blades at lv1 mastery each. You can then unequip Dromarch and have all thunder blades on her and Rex). It could also be useful if for whatever reason you decide you like other blades more than the default ones.

And that's that. This is also completely reversible, after whatever you wanted to do is done you can simply have the driver equip his default blade like nothing happened.