The best way is probably to renew the SEP class rating and then do the differences training for the Microlight.
A smart way to do it if you want to fly the C42 is to find a C42 certified to VLA and renew your SEP on that then if you want to fly micolight simply find a C42 which is restricted to the Microlight Category.
They are both the exact same aircraft just the paperwork is different.