To answer your question, we design our interfaces to be as backward and forward compatible as possible. There are rare situations where something will change but those are pretty rare. With that said, in most situations you should be able to take a WSDL from v9 and use it on v11. Just keep in mind that you won't have any of the new methods that were introduced between the v9 and v11 releases so you would be only able to change on the v11 box the features exposed back on v9. The same goes if you go in the other direction by using the v11.x WSDL files on a v9 device. In that situation, you may incounter some "Method Not Found" exceptions if you try to call a method that wasn't on the previous version. Make sense?
One question though: What client language are you using where you need to use local WSDL files? We provide Java, .Net, and Python libraries to help you from having to deal with the WSDL directly.
Hope this helps...
-Joe