The original consultation alluded to there being no ability to transfer (I assume to stop the ridiculous situation we have now, where individual airlines make you change even though they are all JAR and should be European wide).
However Jetset is quite right, you will be able to change back under EASA. I contacted the CAA as I had heard the same rumour. Check out the FAQ 5 on the link that the nice lady in the CAA sent me:
http://www.caa.co.uk/default.aspx?gid=2061
It says you can change state as long as you transfer your medical records at the same time.