existing jira group memberships are lost when a user is authentication for the first time using an external directory

Use case: switching existing installations from internal directory to external directory (LDAP, Jira or Crowd).

Currently if the username does match (local and external), the first time when a user logs in (using external) it will lose that user's existing group memberships.

The allocation of default directory memberships is very useful but it doesn't have to override current memberships.