I just saw this post "Useful Approaches to BIM for Renovations" by Jose Oliveira.
He talks about the approach to modeling a project in an existing building (by using existing documentation, CAD or hardcopy, and filling in the blanks with laser scanning), as well as the benefits of doing so.
Obviously, coming from the owner side, my approach is different for projects. If we have a model (RVT), and we make a small change in-house, or notice an inaccuracy, I update that. If we only have CAD and need to make a small change, I update that. If we only have blueprint documentation of a specific area, I'll draw it up on our CAD plans, using a PDF underlay as a reference.
We don't create these DWGs or RVTs from scratch in-house, they are all done as a part of construction projects. We have budgets for new construction, but, we do not have budgets for documenting existing spaces, so we often have to wait a long while to get updated electronic files.
As appalled as most on the design side are, that I say we're still about 12 years out from completing our CAD to BIM transition, I have to point out that things do not happen fast in Facilities this big. It took us 15 years to get the majority transitioned from paper to CAD.
Our spaces are renovated an average of every 15 years (with revenue-generating spaces being reno'd more often, and support spaces being renovated much less often).
A space being renovated today is prepared in the Revit family, and will be used with Maximo for CMMS (and the existing conditions were turned over to the design team on CAD).
The last time this space was renovated, it was done with AutoCAD, and was used with MP2 for CMMS (and the existing conditions were turned over to the design team on paper).
The preceding 70 years? All on paper.
It does make one curious to know what technology we'll be transitioning to in another 15 years.