When you format the card as internal (aka adoptable) storage, it links that specific card to that specific phone. You lose the ability to easily swap cards, or to even use the existing card in something else. About the best you can do is backup everything you can (and I do mean everything) then see if there's an option to reformat the card as external storage. If not, then you may have to factory reset the phone. Only then can you swap the cards out.
I'm not too familiar on the exact procedure to undo this because it's never been a recommended practice in the first place. Adoptable storage has a number of drawbacks, including risking early corruption of the SD card. You are best to leave them formatted as external storage, and only keep independent files (media, documents, etc) on them. Leave your apps on the internal storage. If you have so many apps that this is a problem, I would also guess you're having lag issues (low storage devices usually mean low specs all around) and may suggest looking into upgrading the phone.