[Wavytone]

In practice no, you put them in the bin and buy another one.

To recoat a lens is possible in principle - provided it is not cemented - if it is cemented, it goes in the bin.

- grind the coating off using abrasives,
- re-polish and re-figure the lens,
- re-centre the lens (the result will also be a smaller aperture lens)

The problem starts with the modern multi-layer dielectric coatings based on zinc cryolite, which are harder than glass and can only be removed by grinding them off. The trouble is, the underlying glass being softer will also be abraded and the lens will become thinner as a result.

Now the killer - in doing the above, the lens will be thinner than it was, and also smaller in diameter. The thickness matters for optimum achromatism - change that and the lens will not work as well as it did. Theoretically you might be able to tweak the design parameters to find a solution that fits with the glass you have left, but it won't be optimal - ie the lens is never going to work as well as it did before.

Lastly the cost of doing all this exceeds the cost of a new lens, which is why no-one does it in practice. Cheaper to get a new one.