"Grandfathering" means that the restrictions part of the new rules applies only to newly created groups. No capabilities are removed from groups which already exist. Specifically, your existiong subgroups will be fine, and your current basic groups can continue to create new subgroups.
I think of my own grandfather with this word. He started to drive before there was any driving test in Britain, possibly even before there was a driving licence. He was never required to pass a driving test, even 50 years later: his right to the driving licence was grandfathered in.
? - Mark (not that Mark, of course)
Mark Irving