....EASA had confirmed grandfather rights for IMC holders....
In the world of EASA nothing is 'confirmed' until it is issued in print (and grandfather rights for IMCR holders isn't). Generally speaking, EASA bureaucrats are a bunch of corrupt liars who will
say anything to pacify the angry mob.
is it correct that the IMC will need to have a current renewal at the 8th April date?
Again, nobody knows because nothing is in print. However, on the basis that EASA will not convert a National ATPL into an EASA ATPL unless it has a
valid multi-pilot type rating attached, my guess is that,
if there are to be grandfather rights, they will apply only to valid ratings.