Your concerns are justified, FEMA does change it's map but the Seller's have no way to guarentee that the change will benefit the property. FEMA maps are doen by sattelite imagery so there are times where properties on the outer edge really aren't within the floodplain. Having a surveyor do an elevation survey may prive there house isn't actually within the designated flood plain and then paperwork can be filed with FEMA to have the proeprty exempted from the required flood insurance. You can still insure it (and probably should to be safe) but the rate will drop significantly.
The sellers are offering a reasonable solution, "they will pre-pay two years of your insurance" however ultimately you have to decide if you're comfortable taking the chance of being saddled with flood insurance forever. You did not ask about getting out of your contract, but if you do decide to terminate you should consider what financial/legal ramifications (if any) there may be.