So if I've got an IMCr on a CAA lifetime PPL that means I must convert to a Part FCL licence before 8th April 2014? (I knew I would have to do it one day, just checking the date.)
Yes - if you wish to continue to use IMCR privileges on EASA aeroplanes.
Does this apply for an IMCR renewal (mine expires Sep 2013) which isn't a "change" and my JAR PPL in Sep 2014?
There are a few guys in our group with lifetime CAA PPL's who seem completely unaware of this so I imagine there will be many more who are in the same boat as Gertrude.
You may revalidate your IMCR in Sep 2013 and use them on both EASA aeroplanes and non-EASA aeroplanes unti Apr 2014. Therafter you won't be able to use IMCR privileges on EASA aeroplanes until you convert your JAR-FCL PPL to a Part-FCL PPL. Then, as it'll still be valid, your IMCR will appear as a valid IR(R) on your new lifetime Part-FCL licence, with a validity expiry date of Oct 2015 and will again be valid on both EASA and non-EASA aeroplanes.
If losing 5 months of the JAR-FCL licence time for which you've already paid is of more concern to you than losing 5 months of IMC privileges on EASA aeroplanes, then you can wait until Sep 2104 to convert your licence. But it will probably be a lot easier if you convert before Apr 2014.
Although AOPA and other organisations have been attempting to keep their members aware of the changes, it is recognised that not everyone chooses to be a member of such organisations - and I share your view that there will very probably be a huge number of 'unaware' pilots flying EASA aeroplanes on UK IMC Ratings after Apr 2014......