Hi Tammy,
[Begin soapbox]
The best practice is to start from XML and use a middle-tier (aka Web Service) to map to a database. Then you don't have to try to deal with the special schema bindings that InfoPath uses to map to SQL. My guess is that there's some bug in InfoPath which complains but removes the namespace when you try to change the binding. Next time it works.
[End soapbox]
While we try to get people to use best practices ...
Random idea: try convert data source. Go to Tools -> Data Connections in your 3rd form and change the data connectino so it points to the new DB (maybe you already did this). Then go to Tools -> Convert Data Source. Not sure that will work, but it's worth a try. If you can't modify the main data connection, there's always ...
If you if you have no other choice, you might consider cracking open the XSN package (rename as .CAB and extract the files) and finding and replacing the old bits in the manifest.xsf and the schema XSD files. This is not recommended because it's highly prone to messing up your solution. Once you fixup the main data connection in the XSF to point to the new DB. Right click on the modified XSF and choose Design. Then, go to Tools->Convert Data source.... just an idea, but worth trying ...