Changing android signing key password

Does anybody know how (and if) you can change the password used to sign android builds? We have a live app, but want to change the signing password. The problem is that we don't want to force our users to reinstall the app (if we use a new key users won't be able to install the new build as an update to the original app). I've looked all over the android documentation, but have been unable to find anything even related to the subject (except for making a new key...).