you have 3 options,
1) you could either just leave all the existing in the "basic" group, on there next post (assuming there aready over the limit you set) they will get auto promoted to the new group.
2) Put all your current members into the "advance" group, so the new rule applys only to new members.
3) Leave ALL members in the basic group then run the following simple query UPDATE ibf_members SET mgroup = new_group_id WHERE mgroup = OLD_GROUP_ID AND posts > x
obviously replacing new_group_id, OLD_GROUP_ID and x with what the veriables should bee :)