You can choose to use your GUC only for D1 or allow it to be used for Premium Select OR D1. If you are upgraded to PS then your GUC is considered used (and it would require another GUC to upgrade from PS to D1). Most folks here reserve their GUCs for the D1 upgrade and not the PS upgrade (i.e., they use it for a Y to D1 only).
The whole thing is a bit silly, but it is what it is. Unfortunately Delta has removed C+ from PS-equipped planes so it is a bit of a gun-to-the-head situation since you're otherwise stuck in true Y.