Hello,
Yes, each new version of LabVIEW only carries the capability to save for one version previous. So, you would need to save the 6.1 VIs for vesion 6.0, and then from LabVIEW 6.0 save for 5.1. If there aren't too many VIs, one of our engineers here would most likely have no problem with making the conversion for you. You could email NI with them to make it more private, if you'd like, rather than posting them on the site. (To create an email request, go to www.ni.com/ask).
Hope this helps!
Liz Fausak
National Instruments