Is it possible to partition the harddrive with a second installation of MacOS, and then delete the original partition?
I've solved this problem by taking a completely different approach. I used Disk Utility to clone my internal SSD onto a new partition on an external SSD. I am now going to restore my internal SSD back to factory (by restarting, then CMD + R), then update it to 10.12. This way there is only a single partition with a fresh install of 10.12 on the internal SSD. I will still have my old bootable install of 10.10.5 and files on the external SSD, in case I need to access files or restore it back completely.