So your OP says: Uninstall from Play Store, install from GitHub. That is what I recommend against as it won't make a difference from the perspective of Play Store. It just means messing around with your wallet app and those who forget to make a good backup will inevitably lose funds for nothing. The Play Store doesn't care if you used it to install an app or not. It lets you uninstall apps you did not install using it and it lets you update apps oblivious of the sideloaded app not being signed by the same key. The Play Store app would just issue an obscure error if the signature was from a different key.

Unless you de-google your phone in the process, uninstalling is completely unnecessary and only dangerous.

Unless Phoenix is planning to force-close on its US users, there is no need to follow their instructions to close channels before a date neither. I hope they stir drama for drama sake only cause in panic is when people lose their funds. 

 I just did myself the test on a separate device with Google play + an old Phoenix wallet.

Had like 2000 sats in it, legacy Phoenix even.

1. Saved the seed
2. Uninstalled from Google Play.
3. Download from Github the apk and install it
4. restored with seed
5. Funds were there just fine.
6. In Google Play account do not appear anymore as installed 

 That must have changed then. But still, even if you keep it installed on Google Play: installing updates via sideloading should just work. The uninstall step is unnecessary.