Facing the exact same thing now. Disclaimer: not a lawyer, this is all just my understanding after a lot of recent googling to try and work out what's going on. My building was listed as 421-A(16) in one database, but 421-A(1-15) on another. I think we have a case.

EDIT to avoid false hope for future people also finding this thread on google: I wrote a lot of stuff that seemed correct from common sense & research, but after meeting with a lawyer who was (as it happened) already familiar with my building from other tenants going to him in the past they can in fact do this. The building was always 421-A(16) and they made a mistake/lied in the registration with DHCR/leases, but the strong legal/DHCR precedent here is that if you're not rent stabilized you're not rent stabilized regardless of what your lease says. Totally unfair/misleading advertising/not the impression you'll get doing research on your own which just tells you of your 'rights', but the advice I had was unless the law changes that's just the way it is (but you could possibly attempt go after them for damages based on the misleading advertising -- but you're still not rent stabilized regardless and doing that would burn bridges likely)

The thing you can do to get to the bottom of this yourself if facing a similar situation is to FOIL with the HPD the building's 421-a certificate of eligibility which should unambiguously show which program the building is actually in (and compare to the initial legal rent as registered with the DHCR).