Reformatting is only warranted if there is actually any damage to any of the partitions on either of the drives. Moreover, reformatting will not reassign a drive letter. It will just create a new partition for the Operating System to write to while causing (in this case) needless data loss.
If you want to stay away from modifying the settings in the CMOS or BIOS, the best thing you could do is physically swap the drives from cable to cable. That should cause the BIOS to recognize that there was a hardware change and a new "Primary Master" drive.
My best advice to you is to leave reformatting as your LAST resort.