When I did a samba3upgrade all the users and groups transferred, but none of the groups were populated (except "Domain Admins" and "Domain Users"). Repopulating them wasn't a big issue, but it would be nice if this happened on the upgrade.
could this be improved?
Probably you didn't change the group type. Look here: https://wiki.samba.org/index.php/Samba4/samba3upgrade/HOWTO