Sorry, but you're misunderstanding a bit how unlocking works. First, a little 'hierarchy': Google licenses Android to the manufacturers and can sell 'pure Android' phones directly to users. Manufacturers tinker with Android to add their own features and sell the phones to carriers or directly to users. Carriers, in turn, tinker with the software some more and add some bloatware to put the cherry on top, and then sell the phone to you.
Google-sold phones, like the Nexus line, get updates directly from Google because the software is 'pure Android', just like iOS does.
Phones bought directly from the manufacturer can work on any supported network, and are therefore called 'Factory Unlocked' phones. Since they come from the manufacturer (in your case, HTC), they cannot receive the updates directly from Google because they modified the code to add in their features, UI-customization, and hardware-specific tweaks. So, Google gives the manufacturer the newest version of Android and then the manufacturer starts modifying it again to fit in their features. Sometimes, manufacturers for whatever reason decide not to update or take ages to do so.
When you get your phone from a Carrier (in your case, AT&T), they take the phone from the manufacturer (who already modified the core Android experience) and modify it some more, adding bloatware usually in the process. Also, they 'lock' the phone to work in their network only. This means that for them to update the OS, the carrier has to wait for the manufacturer to update their version of modified Android and THEN modify it again to suit the carrier's needs. Needless to say, if the manufacturer takes time to update and they only handle their own products, imagine how long carriers can take if they have tons of other companies involved. Sure, they're not doing it alone and they do get support from each manufacturer, but you get the idea.
Now, a phone already bought from a carrier CAN be unlocked, but this would only free the use of another carrier's network; the software remains the same. That means, unfortunately, that you're tied to the carrier's update cycles.
A way to get around this is to flash another ROM, or Android code that's already been modified and (hopefully) optimized to use with your phone by another programmer or dev. Usually, you can get an updated version of Android with these ROMs or tweaked versions that add features or remove annoying ones.
Now, while this is great for customization and making your phone your own and being all about flexibility...this also means a ton of different options. Cue in the infamous 'Android fragmentation'. It can get scary if you're a new user.
That being said, HTC is quite committed right now to updating fast (they do need the leverage to get out of the financial pit they're currently in), and are also 'encouraging' carriers to update as fast as they can, and are also willing to help. So even if AT&T has a reputation of slow updates, that might not be the case for the HTC One (or the Samsung Galaxy S4, which is also being pushed really hard for updates).
PS: If you REALLY like the HTC One, but really need the 'directly from Google' updates, then I suggest you sell your current phone and use the money to buy a "Google Edition" HTC One. Just remember, you lose the HTC-One-specific features. You just get the fancy outside, nearly cutting-edge hardware with a 'pure Android' experience.