It's a pain in the ass. I have came back the other way. You will lose your IMCr as it can only go on a UK licence. I was lucky in that I held a UK licence originally and still had a national ICAO licence with the IMCr on it so git it all back in order on transfer.
You can only have one Part FCL licence at a time so you will lose your UK one completely. You will need to get a UK national licence to continue to fly UK Annex II aircraft.
Depending on the state that you go to your examiner may not be able to sign your licence which means a re issue every time you renew or revalidate a rating. This was the case with my last state and with 14 type and class ratings on my licence it took some careful coordination!!