>My company has a legacy system running pure VFP8.
>One of our client wants to move to sql server as a back end.
>We thought about rewriting the app using c#.
>But have decided to use the same VFP code and simply adding SQL server capability...... with same table structures.
>
>So, the app can run in full VFP mode. or full SQL SERVER mode.
>
>Can anyone make an argument AGAINST doing such thing?
>
>Thanks in advance..

For many years I worked with VFP based software produced by a major manufacturer (nameless here but with some imagination you can figure the name)
Faced with the same choice, they chose your option - one code base for two back ends.
The result was a disaster.
Maintenance classes - CRUD - were OK - they used classes as Tore suggested - but the reporting procedures were nightmares. Since the queries had to conform to two backends, neither was well supported. In many cases they wound up writing two versions.
We supported the code and it was painful just to look at it.
Several people involved told me that in retrospect, they should have produced a VFP/SQL Server only product to run side by side with the .dbf product.

This might not be relevant in your situation, but it's something to consider.