Elastic Beanstalk - RDS configuration issue with Ruby on Rails stack

I have an Elastic Beanstalk's RoR 2.2 stack on AWS. I configured it with an RDS instance (I set it up with elastic beanstalk). But I have a problem with my configuration. The problem is the RDS_HOSTNAME, but i'm unable to modify it, either from the EB GUI or the command line. And I can't make this change persistent, when I try to edit the value directly from the EC2 instance (with SSH).

I get the following error if I try to deploy or if I run a eb setenv command :

INFO: Updating environment Frontend-Production's configuration settings.    
ERROR: Updating Auto Scaling group named: awseb-e-xxx-stack-AWSEBAutoScalingGroup-XXX failed Reason: Template error: DBInstance zzz doesn't exist
ERROR: Service:AmazonCloudFormation, Message:Stack named 'awseb-e-xxx-stack' aborted operation. Current state: 'UPDATE_ROLLBACK_IN_PROGRESS'  Reason: null

Now i would like to dissociate my RDS instance from the EB stack ? or at least how can I change this RDS_HOSTNAME value without terminating everything in my stack ?

Thank you !