Hi Nitish, if you upgrade just one partition and the switch reboot with the old one (in case of wrong configuration), it will use the old image. Since there will be a "downgrade" from 15.x to 12.x that could cause some issue.
Even if it's not mandatory to have both partitions using the same version, it's recommended (at least after you assure the new image is working as expected).
You can upgrade just one partition and work with that new image for couple days and if no issue is seen, than they just upgrade the other partition. This will help to revert back to the old image in case of any issue.