Since you are already rooted, flash a custom recovery like cwm or twrp and do a nandroid backup. Then relock and install stock recovery--there are threads on this, I have not done it. Rooted first day I got phone and started running custom roms. No need to unroot, OTA may do that anyway--that's how I remember reading it.
There are exact steps back thru this thread--this is just the basics I remember reading about
Then you can unlock, root, flash custom recovery and restore your nandroid.
I will say some have had trouble re-unlocking. So, I would flash cwm recovery and one of the custom roms for EU that has the update and some additional features.
No matter what you do, I would flash cwm at minimum, nandroid and copy nandroid/sdcard to pc