You sum up my position too!
I can't see a reason to convert to JAR before July until the IMC migration issue is resolved.
Ah BUT - the Catch 22 is that the CAA says it will not be issuing any more JAR licences after 1st July 2012. And it seems that IMC ratings cannot be attached to EASA licences. So if we do need a JAR licence to preserve our IMC ratings we have to get one before 1st July 2012.
I doubt it.
PERSONAL OPINION: If you have an IMC rating in a JAA PPL now, it will last (in EASA aircraft) until April 2014 OR until the JAA PPL expires, OR until you apply for any new rating in the JAA PPL from July 2012. If the JAA PPL expired in say Dec 2012, you would receive an EASA PPL, which could not have an IMC rating in it, but I think/hope you will be issued with a CAA UK PPL at the same time, with an IMC rating in it, to take you through to April 2014 in EASA aircraft. One reason I think that is because the CAA changed the ANO in 2000 to stop themselves issuing a CAA UK PPL, but their recent consultation on changing the ANO for 2012 includes letting them issue CAA UK PPLs again, and I think that is part of a solution to buy more time to sort out a permanent solution.
Still personal opinion, I think/hope that if you apply from scratch for a new IMC rating after July 2012 and before April 2014 (or until there is some final solution), I think/hope you will get it issued inside a UK PPL, either one that you have now, or in a new UK PPL they will issue with it.