This is probably obvious, but note that if you've already published a version using a second key, then going back to the first key will cause another disruption for your users. You should just stick with the last key you used.
FWIW, this is all explained in the Channel Packaging and Publishing doc, under Packaging Your Application. The rekeying process is also explained there.
* Applications can share data via registry entries if and only if the applications are packaged with same key
* New versions of applications will have to re-link if they are packaged with a different key
* It is critical to save the password and keep track of the key that signed an application. If the password is lost, you will not be able to sign any new packages (including new versions) with the same key.