When playing XCF and building an Advanced Intelligence Center over Intelligence Center (which provides space for5 researchers), with all the researchers full at work, I got to -5 available research space. So I suppose the scientists can continue working on their on-going projects in the living quarters even if the lab goes under construction.
I recall seeing this before as well. If you reassign scientists, you can no longer go negative. But I do have a recollection that some others things are checked in other build-overs (IIRC for example building over a facility that provides storage capacity requires reducing the used storage capacity first).
I wonder if it's intentional that this is not checked at build-over phase, and prevented - requiring you to de-assign the scientists first?