Perhaps this is a request to generalize Change Method Parameters to Change Method Signature.

Basically, I just encountered a situation the IDE should be able to do, but there is no way to do it.  I had

public <T> T performQuery (ObjectIdentifier id, Class<T> returnType);

and would like to change the signature to

public <T extends Identifiable> T performQuery (ObjectIdentifier id, Class<T> returnType);

It would be useful to have a refactoring that allows this - Change Method Parameters seems like the logical place (with a better name - Idea's "Change Signature" is clearer anyway).

And generally, refactoring a return type to be a supertype or subtype of the current return type is something the IDE could do.