You misunderstood. If you already choose e.g. monthly Ultra, there is no option to change for the one-time option or to the yearly subscription. You cannot change it, just with cancelling your already active subscription and go with another option.
But I think it’s a limitation of the Play Store but I’m not sure.
Yeah, like linux doesn’t do this shit all the time. Permission denied always. I’M YOUR FUCKING GOD, DON’T EVER DARE TO GIVE ME PERMISSION DENIED.