I believe that if the appraisal stip is passed then the buyer cannot walk under the terms of that stip. However if you sign the FHA amendment there is also an appraisal contingency in that. So if you have signed the FHA amendment then you would now be subject to the appraisal contingency provided in that amendment but not the original appraisal contingency.

You definitely need to consult with your agent about the possible scenarios involved.

Can you ask your agent why the FHA addendum has arisen now?
On your original purchase contract, was your buyer stating conventional financing?
Have you or your agent seen anything in writing from the lender that the appraisal was satisfactory with no conditions? If not, can you get that? Do you have an attorney, what does s/he say about the buyers' options?

I've had a few of these this year - for one reason or another, the lender and buyer opted to convert the loan to FHA (declining credit scores, cost of PMI, needing to put more $$ down to get the conventional loan). In all cases, the lender required the parties to the contract to sign the FHA addendum for underwriting. So, in those cases, it had little to nothing to do with the appraisal, and everything to do with the buyers' ability to finance under a particular type of loan.

You would want to make sure that you're not summarily agreeing to any additional costs or fees on behalf of the buyer if the loan is now being converted to an FHA loan; also I'd check to see if the appraisal that was already done was an FHA appraisal or not.