I created my RDS Subnet Group via CloudFormation referencing a parameter ProjectName

Problem now is CloudFormation says it cannot find my subnet group:

DB subnet group ‘AbcDef’ does not exist because its actually abcdef … how can I resolve this?

I tried looking for a toLower function but seems like theres none?

The other option appears to be recreate the stack?

Unfortunately everything you do in CloudFormation templates is case-sensitive including property names and parameter values. You may have to recreate the stack.

As you correctly pointed out, there is no Fn::ToLower function. If you really want to achieve what you are trying to, the only way to do it as of now is create Lambda backed custom resource which basically will convert your string to lower case and return it but it is not worth doing it as there are plenty of challenges you will come across when dealing with custom resources.