For domestic flights, I often buy a b fare and upgrade. Yesterday I was plotting a domestic flight, and there was a new option: First/global first. For the flights I looked at, the price was materially lower than a b fare, and gave me an a fare.

I'm hardly complaining, but I'm wondering if there is some disadvantage to the First/global first option. There didn't seem to be any explanation on the web site of this change.

Not uncommon for a discounted first (A) to be cheaper than Y/B fares.

The Y/B fares are likely to have fewer restrictions -- such as lower or no cancellation / change fees.

Also some organization will only cover economy fares even more expensive than discounted F.

Could this be a M fare? I'm 1K and get an upgrade on M fares if it is available. I was offered this on EWR-DFW the other day and I calculated the buy-up and it was exactly the difference between my H fare and M fare.

If the OP was searching for flights via the website (I'm reading the plotting they did as such) then an M-fare would still show as United Economy on the search results page (with the Premier 1K Instant Upgrade symbol visible). It isn't uncommon to see A or even F fares show up on that page when searching for economy on close in flights.

I've seen really cheap H-up fares that seem to require H availability (and maybe something like PN, too?), but book directly into A. They seem wrong on two counts:

1) They can be cheaper than M fares, so feel like yet another way to undercut 1K benefits.

2) The receipt shows that a First Class ticket was purchased, so they don't work to defraud your corporate travel department as the Y/B/M "instant upgrades" are designed to do.

It's all designed to generate additional incremental revenue for the airlines. If you'll buy the B/Y fare without looking at the A with the "hope" of getting an automatic upgrade, the airline wins. The entire reservation/fare engine is designed to extract as much revenue as possible.

On some markets, the discounted first class fare (A) is cheaper than full economy fares (B or Y). This is the case on the IAH-LAS market. Always check both before booking.

These fares do book directly into A. They do not require H availability. The fare basis code begins with the letter "H" (such as HUAUPN) but there is no requirement that a fare basis code start with the letter of the fare class it books into.

It doesn't have anything to do with 1K benefits. The cheapest H-UP fares are non-refundable and often have advance purchase requirements. M fares may or may not have restrictions. They're being sold to different markets. And on many routes there are no restricted H-UP (A) fares, only unrestricted ones which are usually a little more expensive than B. It's probably safe to take the foil hat off now.

Actually, some of them seem to be filed as "Economy" fares, and there's a good chance the corporate travel agency system will still pick them up as Economy (based on the first letter of the fare basis code). Probably depends on the agent.