The older labs, for the most part, do take time to rebuild. (Detailed in House-Hunting thread.)
Scylla's old lab (taken by Angus) will take two seasons to get usable: one to get from a -5 penalty to a -3, and a second to erase the penalty entirely.
Cainneach & Cináed's (Corbin) and Sylvanus's (unclaimed?) will take one season to get from -3 to no penalty.
All of the other labs (Curwen, Doineann, and Drystan/Corvus) are technically usable as-is, although the new owners might want to take time to reconfigure them.