Paul,
Switching Datasessions on the fly is a great way to GPF VisFox!
(Thats why its not recommended),

The best way I have found is to pass the DBF name to the form, and
reopen it on the other side:

DO FORM MyForm WITH DBF('MYCURSOR')

MyForm.Init:
Parameters cCursorName
USE (cCursorName) AGAIN SHARED IN 0 ALIAS MYCURSOR
* Read/Write to cursor; changes will be available to calling form