I'm trying to figure out whether using Google app signing means the released apk's (on the Play Store) key signature (X) is different to that of a release apk that has not been uploaded to the Google Play Store/Console (Y).
When i manually install Y on a device that has X installed it won't install Y with the message, "App not installed"; I assume this is because the two versions have different certificates.
When I upload Y to the play console and release it to the play store will the user receive this message and be unable to install it without first uninstalling their previous version? I don't want their data to be lost. Or will the certificate be changed in some way by Google app signing si that it matches X.
No theres no issue if you update your app on Google Play store with new version if you are using Google app signing. Google app signing will not create problems when updating app on Google Play store ~ which you are experiencing while testing with release apk.
You are only experiencing this issue:
When i manually install Y on a device that has X installed it won't install Y with the message, "App not installed"; I assume this is because the two versions have different certificates.
..because the app is not installed through play store which uses Google app signing for authentication.
If you want to test the app for the end user use Alpha release and later promote it to the production when you are done with the testing in alpha release.
Read more on Release apk testing on play store and Google App Signing