BlackBerry - the app is already in the store; i can change the code signing keys

We created an app for a client some time ago, signed it with our company keys, and put it on the BlackBerry App World store for them.

We have since terminated our contract with the client.

Now they've done some more (at home) dev in the app and asked us to sign our keys so they can sign it. The answer to this query is a very loud "NO!"

My question is:

• If they apply for a new signing key and sign a new version with that key, will they be able to download the new application and replace the current one in App World? i.e. as version 2.x

(Or the key identifies the app, for example on Android, you have to sign your apps with the same key in order to send the new version)