>> is it possible to change ldap to rdbms for authentication?
Yes. For 'Authentication', it's the same as configuring 'Manage Authentication' feature.
But the main challenge is pulling user's information from RDBMS with Cache Refresh. We use 'RadiantOne VDS' to do this job for RDBMS, which is not included in community edition. That is included in [Gluu VIP Support](http://www.gluu.org/gluu-server/pricing/) Editions.