How y'all versioning web services?

Let say I publish a web service and have 100 clients calling its methods. I assume I am free to change the implementation as long as I dont change the interface.

Now, lets say I want to change a method signature. Do I have to publish this interface as version2 on a separate URL? I assume that if I *replace* the old interface, everything would break. I would have to regenerate the proxies on all the clients. Is this correct?

TIA
-Dave