Are there available any estimates on how long it would take FEMA to implement the use of a 500 yr flood period as the basis for mandating NFIP, or , alternatively, use of "future-conditions" 100 y flood in lieu of historic 100 yr flood maps?
I'm not certain but I believe FEMA does allow the use of "future conditions" now. Have you checked their website recently ? The so called 100 year flood was a legislative compromise when the NFIP went through Congress so I guess the answer to your other question is that it would take an act of Congress to change.