How exciting to have a bat box that's been adopted by bats - Lots of folks put up boxes, never to have them get adopted, so you've clearly got a winning combination figured out! 
Are there bats using that box RIGHT NOW? In your favor is that the bats shouldn't yet have any offspring, so there should be low to no risk of having moms abandon pups. 
You do NOT want to remove and handle individual bats - far too many risks for you and the animals. I think if you add the new box right next to the existing/old box and remove the old at night after the bats have flown out to hunt, that there's a better chance they might go ahead and roost up in the new box. You could use some fruit-tree or window netting to create a check-valve on the exit to the old box to make sure bats cannot re-enter the old once they've flown out. Pages D18-19 of this old publication (attached) talk a bit about checkvalves, in case that helps. 
I honestly don't have experience or advice about what (if anything) you can or should do whilst painting that wall if bats are in the box while you're painting. I advise calling ODFW and asking to speak with one of the non-game or bat white-nose-surveillance team members to benefit from their knowledge - They might have even better recommendations to suit this situation. 

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