If you use your nodes efficiently knowing you may do this you can add nodes and then take the key mix from 1.0 to .50 and mix the node in. Versions with totally different looks and node structures not as easily done.
Using nodes means not using versions... which goes against the concept of using versions :-)
I guess the best solution is to create a 3rd version, make it include the nodes from both versions (using Gallery->Append To Node Graph or a lot of Add Node + copy-paste), and then restructure the node tree in a way that'd facilitate blending with a parallel or layer node. Not exactly quick.
I don't think the issue I raised relates much to Sascha's issue, but the solution for both cases is to expand the scope of nodes beyond a single version. Also consider this scenario: let's say you have a grade on 85 light-saber shots. The shots and grades are different, but the light saber components are the same. Then production wants to change the color of the light saber in the movie...
I would just make an extra video track and blend that in normal mode .
But I agree, the whole picture in picture topic needs more attention at this point.
I would live to see a cascading node that just takes what I have and puts in on top .
So I can use a clip from a different time, loop a portion of it and superimpose it over..tracked.
Vey useful to clean up sky's, signs, roads and sudden boom drops.