O2 sensors. Replace one or all?

[RandallNeighbour]

My local shop is suggesting I change out all four O2 sensors because one is sending a message to the computer and giving me a check engine light.

Any advice? My boxster is a 1997 model with a new engine in it (probably no more than 5k on it) I have no idea if they changed out the O2 sensors when they replaced the engine... that was before my time.