the old firmware patch is modifying some files in /system.
you could try restoring only the data partition,but i believe that it wont boot. im fairly certain that amon wipes all partitions automatically prior to a restore,so after restoring /data,you wont have a /system or a /boot to make the rom work
i could be wrong,tho... trying it shouldnt cause any harm. if it doesnt boot,then youll just have to pull the battery,and relaunch recovery to fix it
after some thot,here is what you should do:
-reflash GB backup and take OTA
-restore current backup(dont reboot after)
-wipe cache and dalvik cache
-reflash rom
-reboot
relflashing the rom
without a data wipe should preserve your setup,and replace "/system with old firmware patch" with fresh /system install,ready to run on new firmware