Just some experiences to share:
Was on stock rooted EL29 GB. Flashed to Agat's latest themed FC22 with only doing a cache and dalvik wipe. Actually worked pretty well after I did some cleanup. However, there were a few glitches so I wanted to go back to my nand of EL29. I was too lazy to Odin back to a EL26/29 rom so I just took the plunge and restored my EL29 nandroid using this recovery. There was no problem.
Long story short is I belive that flashing back and forth between GB and ICS rooms is fine as long as both are TW based. Once you throw anything AOSP in there things have a potential for brickage. Mind you, this is only my experience.
Sent from my SPH-D710 using Tapatalk