We operated our site in normal ("single user") mode before switching to network (multisite) mode. So we have pre-existing users. We also have some recent users who registered using the standard login page (not the network signup page). Apparently all these users are not recognized as "site wide" (or global) users. If they try to register on another site in the network, they are denied and told their info is already in use.
How can I "promote" all such users to global users, or site wide users?