The three major service providers: AIS, DTAC and TrueMove (H) all kicked off 2100 MHz 3G services in the Kingdom this past week.

Additionally, TrueMove H also launched 4G/LTE on 2100 MHz (Band 1) in metropolitan Bangkok.

For the most part these services are targeted at post-paid customers, and details are just starting to become available. I don't think most tourists/visitors will be impacted, at least over the next ~ 90 days.

I think I'd continue to recommend TrueMove H and/or DTAC/Happy for pre-paid voice and 3G data - assuming your phone supports 850 MHz/3G.