I would recommend you just stay on 12.3 for the time being. Support for 12.3 has been extended until at least 6/30/2018. If you are going to run any version of 15.1 on legacy (non-ELS) EX switches, you should use a minimum of 15.1R6-S2 at this time. I do not know if you would see the same situation with 15.1R6-S2.
I do assume for your 2-member EX4550 VC that no split-detect was enabled, yes? Without this, you will see the VC split upon a reboot. Reboot is part of upgrade process.