The actual solution is to move everything back to the phone (it might have to be done in stages if there's not enough space on the phone) and store it off-phone. Then switch cards, format the new card to internal. When it's done, install all the backed up apps and restore the backed up files. (Data might be a problem, since not all apps make their data available - which is why I don't consider this a good idea for any but the smallest phones. 4GB internal storage? Maybe. 32GB internal? No.)
BTW, a factory reset wipes ALL storage, so there's probably nothing on the card. It may show that much data in use, but that's because the factory reset doesn't reset that for non-standard storage. Format the card as FAT32 in the computer and use it for something else.