My only input here
is going to be to say that almost everybody who takes a significant pre-emptive action in fear of threatened future legislation
if that legislation appears unpopular and exceptionally aggressive yet bringing no personal brownie points to those forcing it through is going to lose their money.
The most obvious examples are of people who went N-reg (in preparation for the FAA IR) and then, when the DfT threatened to kick out N-reg planes after 90 days' parking they got scared and went back to G (£10,000 perhaps??) and thus have scuppered their IFR privileges for a few years and possibly much longer.
The "UK IMC" issue is a similar sort of thing.
As others say, things are invariably grandfathered into something else...
I would definitely do the IMCR.