You can open form as a table. (I suppose you know that form's description is stored in a table: SCX file has dbf structure and sct file is SCX's fpt file)
The information about the form is stored in 3rd row in the table - check ObjName field. You can change the content of the fields Class and ClassLoc. DON'T WRITE IN BASECLASS FIELD. If you leave ClassLoc field empty FoxPro will ask you to select the vcx next time when you try to open the form.
Close the table before trying to open form for editing.