My interpretation is that the grandfathering would immediately cease to be in effect because the new proposal affects all group members. Even if a group itself is grandfathered in, members may have to start paying to join other groups. Nobody who created a groups.io account thought that was going to happen.
In case that wasn't clear, the grandfathering *would* be in effect if the original owners-pay proposal takes effect instead.
While we don't know if this is the last time anything gets grandfathered (due to some change) or not, grandfathering would not cease to be in effect for existing grandfathered groups regardless of solution; per Mark (4th topic message) they would stay legacy and would not be subject to the membership-cap+charge plans presented; they will stay as they are, no caps, and any Groups.io member can join them for free. But those groups' members would indeed be subject to the new rules when they attempt to join non-grandfathered groups, but then again we don't have anything like user grandfathering.