Flood insurance is a form of property insurance. It is not a closing cost subject to a good faith estimate increase cap of 0% or 10%. Therefore, there is no changed circumstance warranting a revised LE. You should be good, since you had no reason to believe flood coverage would be required at the time the LE was issued.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8