Originally Posted by MSPeconomist

Just make sure that any C+ upgrade doesn't consume a RUC. On certain international flights, it can easily happen: apply a GUC and C+ is available, then if D1 upgrades open up later, DL will ask for a second GUC. This is supposedly the new DL policy, especially as premium economy cabins are introduced as not only a separate fare class (W, WP, etc.) but also a separate cabin class.