As you probably know, HTC One came out in a few variants. I described these different versions here and here. To sum up:
The important part is, that users with U variant (GSM only) can use custom ROMs based on UL (GSM/LTE) RUU. It is already confirmed by one user, who flashed custom ROM based on 1.28.401.7 m7_ul software on his m7_u unit. Don't be confused - this probably won't enable LTE support on your GSM unit. We are not yet sure if it's possible at all, but it's very likely that once we have S-OFF and SuperCID for HTC One, flashing entire RUU from m7_ul on m7_u device may enable LTE support (of course your network provider has to support LTE as well).
I will keep you posted once I have some more details.
Thanks goodman_east for sharing some info with me!