New logbook is fine, just don't lose the old one.
If you put microlight and SEP (the new name for the old "group A") in separate columns, you'll be fine. Just make sure they're totalled separately. It's easy to tippex or make stickers on a couple of column headings for stuff that you'll never do, like ME/night, whatever. Equally if you prefer separate logbooks, that's fine too.
G