Old Play Games Services with new app

Hi everyone !

I'm currently working on some old project of the company I'm in, and they forgot the keystore of the project to upload the fixes I did. So I created a new game in Google Play Console (with new package name, etc...) and tried to link the old Play Services to this new app (new linked app).

But when I try to copy resources to Unity, it has the wrong package name (aka the old one), so I can't copy these into Unity without errors. I tried creating a new Play Services in the console, but can't add the app since it's already linked to an other one. I can't delete the old one as it's already published...

Is there a way to make things work?
Thanks !

(I really hope it is understandable as it's not my native language)