Downgrade an apk with different signature, without root

I'm trying to do the following things to an Android app without root access: 1. De-compile the apk with adb d xxx.apk (done) 2. Modify some files in /res/xml/ (done) 3. Re-build the apk with adb b xxx (done) 4. Sign the apk (keytool + zipalign + apksigner) 5. Downgrade with adb push xxx.apk /data/local/tmp/ & adb shell pm install -r -d /data/local/tmp/xxx.apk I'm able to sign the apk with my own keypair, but I don't want to uninstall & re-install the apk to install the modified version -- I want to downgrade the original app to preserve the data. However, step 5 produces:

Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE: Existing package xxx signatures do not match newer version; ignoring!]

It seems like downgrade app's old & new version must have the same signature. Is there anyway to bypass this check, or solve this situation? I read this question: https://android.stackexchange.com/questions/132374/how-do-i-force-reinstallation-of-an-apk-with-different-signature-on-rooted-andro ... but it requires root access.