United Airlines will roll out a function on its website by month’s end that allows frequent fliers to see whether they are in line for a complimentary upgrade to business or first class on upcoming flights as soon as they book.

Currently, United customers are only able to see the upgrade wait list — and where they rank — online or via United’s smartphone app up to 24 hours before a flight departs. Fliers who are part of the Chicago-based carrier’s MileagePlus frequent-flier program must call a reservation agent if they want to find out whether they are in the running for a free upgrade before then.

The modified function will become available for the life cycle of a flight number sometime in the coming weeks.

"We want to give customers more assurance than saying, ‘Trust us that you are on the list,’" United spokes­man Rah­saan Johnson said.

United will also reinstate a function it suspended Sept. 7, to the dismay of many frequent fliers, that allowed those booking flights online to see the number of seats available in a particular fare category. The feature will be visible only to frequent fliers with MileagePlus accounts who opt in for it.

Before United suspended it, even fliers without MileagePlus accounts could see the full spectrum of fare types, or codes, for any given flight.

Fares eligible for upgrades were, and will be, marked with the letter R for "rewards," followed by a number representing the total number of seats eligible for an upgrade that haven’t been reserved by a flier paying full-fare price.

Many frequent fliers had used the fare category as an indication of how likely they were to receive a complimentary upgrade on a flight — for example, "R7" would present a better chance than "R1" — which is the reason, Johnson said, United decided to suspend the feature.

The fare categories did not and will not guarantee a flier any kind of upgrade, Johnson said. That’s because upgrades are processed in 24-hour intervals up to 120 hours before flights. Available seats in business or first class are first assigned to frequent fliers who requested upgrades and will pay for them, followed by fliers in the highest tier and then through the lower tiers.

Making the feature opt-in only for frequent fliers, Johnson said, was designed to eliminate confusion among people who may not understand what those fare categories mean or mistakenly think it is a promise of an upgrade.

Johnson said the reinstated feature will include a disclaimer reminding fliers that it is merely an indication of seats eligible for upgrades at a particular time, but one that may change between the time they see it and the time of departure.

"We’re going to be reminding people how the complimentary premier upgrade process works and how what they’re seeing may be helpful, but it is … not a guarantor of an upgrade necessarily," he said.

The suspension of the function caused a stir on online flier forums in the past week, including Airliners.net and FlyerTalk.com, where United representatives had posted to threads discussing the topic.

United frequent flier Chris Summers said in an email that without the function, "We are now left with the choice of just buying a ticket blindly and hoping it can be upgraded or calling in."