Unhappy [Q] I can't change ROM on my gnex anymore :(

Well, I once deleted the /factory folder from cwm - mounts and storage option and lost my imei. I got that repaired from a shop, though not sure what the guy did to rewrite the imei on my phone. But now the problem is that I can't change ROMs - every time I try to, the imei disappears and doesn't matter if I try to selectively restore just the /factory folder from the backup which I have after the shopkeeper repaired the phone. Is there something which I can do so that I can change ROM and yet retain the IMEI of the phone?