Are you in FC already? I've had this affect me (YMMV, of course) in the past:

If you are already in FC on your original SEA-MSP return flight then like SchmutzigMSP said ask to be put on the first class standby list.

If you are NOT already in FC on your original SEA-MSP return flight then just do normal standby for your earlier flight. You may get hosed on a BF upgrade if they do standby after all of the BF upgrades clear and the GA didn't notice you were a plat on standby.

In both cases, since you're OK with taking a Y seat back if it means you get on an earlier flight then be sure to tell them that (be very, very clear!) either at the ticketing counter or gate. Personally, I'd go talk to a TA a bit before the flight and poke a bit to see how full the flight is. If the flight is empty and they're feeling nice, they may just give you a boarding pass without the standby hoopla. Even if it comes down to BF I've had GAs notice that I was a plat on standby and put me directly in to FC. Other times they've call my name and tell me that they can get me on the flight, but there are only Y seats left and if that is ok. Again, YMMV.