Hi,
LabVIEW 7 is backward compatible. There are of course many changes in it but you should still be able to use your LabVIEW 6.1 application in LabVIEW 7.0.
The best way to test this in to install the evaluation version of LabVIEW and try to run your application on it. Please make sure that you have a backup copy of your application.
http://www.ni.com/labview
Feroz