Using improvements to make other improvements obsolete

[John Campbell]

Hmm.

Bumbi has finished building Curtain Walls.
Bumbi is building Airport as suggested by the advisor.
...
Year: 1892 CE
...
Year: 1894 CE
...
Bumbi is selling City Walls (obsolete) for 60.
Learned Advanced Flight. Our scientists focus on Photography; goal is Television.
Year: 1896 CE

So yeah. You can make buildings obsolete_by other buildings, but that appears to only actually get checked and the obsolete building sold off when you learn a new tech.