UA is moving to restrict Economy Plus to either UA/Star Alliance elites or full-fare (Y/B) passengers.

They are starting to offer upgrades at check-in when using Easy Check-In kiosks (from $25 and up depending on flight distance). So far, people are reporting success in asking at the Gate, but it appears UA is briefing their front-line employees on the new policies so expect your chances to get less and less over the next few weeks and months.