- MileagePlus members seated in United Economy can use a single Global Premier Upgrade request to move to a United® Premium Plus seat or upgrade to the United Polaris® business class cabin.
- If you’d like to be considered for a move to a United® Premium Plus seat or an upgrade to the United Polaris business class cabin, we’ll move you to the first available seat. If the first available seat is a United® Premium Plus seat, we’ll continue to look for space in the
United Polaris business class cabin without requiring an additional Global Premier Upgrade.
From:
https://www.united.com/ual/en/us/fly...-upgrades.html