As I recall, VFP up to version 6 had a converter (referenced by _Transport ?) program that would open 2.6 SCXs; save it as a VFP6 SCX; and generate a new SPR file that would run the new form.
The SPR file contained almost what MikeLewis's First post suggested: a line saying Do Form <screenName>
That way, any existing PRGs that had the line "Do <screenName>.spr" would correctly open the form.
There is a slight additional complication: In 2.6 it was possible to have setup code in the form that could, for instance, declare variables visible across the whole form. VFP6 forms can't do that. So, the generated SPR would include that setup code at the top of the SPR.
Chris