Club Golden is nice because you are in the rooms closest to the beach. Last year we had a Club diamond room that was right next to the Club Golden section. We were close to the beach, but far from the diamond pool so we never used it. Almost always ate lunch on the beach so the "exclusive" lunch restaurants didn’t really make any difference.
The previous year we stayed in the Punta Cana section but still had breakfast in the "exclusive" restaurants a couple of times. Didn’t know that they were supposed to be exclusive and no one said anything to us.
Our package for next January puts us on the Punta Cana side again(AC Vacations). We were going to book Club Diamond but there was a price jump from Sunquest that day. Didn’t think it was worth the extra $.
The rooms themselves are all the same. Club diamond guests have beach towels delivered to your room. No need for a towel card. You also get the use of slippers and a bathrobe. Club Golden/Diamond have their own check in-out/guest services areas.
Other than the Premiere section, the beach is just a tram ride(or 2-10 min walk) away.
Didn’t use the tram at all last year.
Not sure about upgrading once you get to the resort. It would depend on availability. They were trying to offload guests to their other resorts for overnight stays when we were there due to overbooking.