Bleeding off the excess H2 and O2 seems as wasteful as throwing away the tank itself. I would suspect that having an extra ton or two of oxygen and hydrogen wouldn't be all that hard to turn into an extra ton of H2O, which the crew might appreciate. Or if they send up multiple partially empty tanks, they could designate one tank as the recovery tank.
The tank purging process would probably be time consuming, but there should no reason to be in a hurry to convert the tank into a different usable space. Conversion is something the crew can do while under way to their final destination (with the reward of having an extra building to live in after they're all done; that should provide incentive to prioritize the task.) I would question the value of sending dedicated construction robots into orbit since the crew is already going to be there (unless the task has dangerous elements due to the residual fuel, risks of fire or explosive decompression while cutting openings into the tanks, etc.)
It definitely limits the main engines to burning hydrogen and LOX, though. There would be no way to purge a tank holding any of the other fuels they might want to use. Imagine if living in an empty diesel fuel drum was the best of the other available options.