Express VI's were introduced in LabVIEW 7.0. So they cannot be converted to 6.1 directly.
You MIGHT be able to do it if you first convert the Express node to a subvi by right clicking and selecting Convert to SubVI, then try to save that back to 6.1. Depending on what the node does, this may or may not work. 7.0 has many new features that just did not exsist in earlier versions, so not everything can be saved to a previous version. Ed
