We want to update an existing Application but unfortunately we have a problem. Our keystore to sign up the app is not the same as the one we used in the previous version of the application. Therefore we can´t upgrade our existing app in the market. Is there any way to get the old key (certificate) from our previous version which is currently in the market place?

Is there any solution to upgrade our existing version with the new one we would like to publish in the store?
What happens to the users who have installed the current version if we delete this version in the marketplace and upload a the new version (with the same name)?
Do they get an update notification or do they have to install the new app?

thanks in advance