United frequently does not make Polaris seats available for upgrades in advance. Any requests for upgrades will then go to a waitlist. The waitlist is cleared first mainly by status, then fare class. There are then other tie-breakers that occasionally come into play.

Silver status with a cheap economy fare will rank fairly low on upgrade list. You can move up by buying PremiumPlus seats. If the upgrade doesn’t come through, you at least have seats slightly better than domestic First Class recliner seats. If you waitlist for an upgrade from PremiumPlus, you will be ahead of everyone in economy except GS on the waitlist. You’ll be behind higher status passengers who also have PremiumPlus tickets.