I might be off base here, and please correct me if I'm wrong, John.....
Erwin,
When you create a new Block, you're creating new topology (faces, edges, etc.). This topology is identified separately from the original Block feature. The blend is "looking" for specific edges and adding the new topology (edges) causes the Blend to require the new edges be defined. In short, I'd expect you to be able to Replace the Block feature, but the Blend will probably fail and you'll need to redefine the Blend edges. The old edges may still show up when you edit the Blend, but clicking the X (deletes the old edge selection) and repicking all edges should get the Blend feature to update correctly on the new Block feature.
Hope this helps explain what might be going on in the background and why selecting new edges is required. Try to keep that mentality at the forefront when replacing bodies, faces, etc. It will help you diagnose model update failures and feature warning messages that might appear in the Part Navigator.
Tim Flater
NX Designer
NX 8.0.3.4
Win7 Pro x64 SP1
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB