I found some papers [1,2] which claim conversions are not prohibitively expensive, though I assume not all office buildings are equally suitable. Open-plan office in particular seem impractical to convert. I wonder how many of the companies that built them that way to save money are now stuck with an empty building no one wants to buy?

[1] https://pdfs.semanticscholar.org/da2d/49a0090e8631ff08eb4813...

[2] https://journals.sagepub.com/doi/pdf/10.1080/004209805003803...