Is it making any noise?

If you hear a clacking when it's running, or the clutch is staying engaged when it should be disengaged, then yes it's possible that it's going bad even though the air is still cold. Another possibility is that, when they changed the expansion valve, they noticed debris being thrown off by the compressor.

Any of those situations would require that it be replaced, even though the air is still cold.

But if it's not making any noise, and if it is engaging/disengaging properly, then I would be VERY suspicious, and at the very least require them to thoroughly explain their diagnosis, and then report back here.