The first question to ask is whether the client should be moved to SQL-Server. IF their data requirements are such that moving to SQL is the right decision, then that should be suggested. i.e., the client needs to have the benefit of turning that option down. Remember, as consultants, our job is to do what is in the best interest of the client. Suggesting inferior technology to meet our (the consultant's) needs is not the way to go.

With this in mind, nobody can provide you with solid arguments unless the specifics of the data requirements of your client are known. It may very well be that SQL is the right choice and that your client may have to seek a solution elsewhere.

A good question to ask is why you think you will lose the contract. Is it for reasons of perception? Or, are there legitimate technical reasons why you think a VFP data storage scenario will/should lose out to SQL-Server? If it is a perception issue, then it is a matter of correcting those mis-perceptions. If there are technical reasons, I don't know how one can counteract those arguments.