Each LabVIEW version can go back one step uing "save for previous". You need LabVIEW 6.1 to save them in 6.0 format. Then you need LabVIEW 6.0 to save them for LabVIEW 5.1.
Unless they use new 6.0 or 6.1 specific features, they should run just fine in LabVIEW 5.1 once converted.
If you want, you can post your VIs here and somebody will probably convert them for you.