When should a sacrificial anode be changed? Ours still has its basic shape, a bit pitted, but still has plenty of surface area. Yet, I've seen people to replace them at about that stage.
Also, any suggested uses for the old one?
You do not say how long the existing one has been 'on' !
When inspecting boats it's surprising just how many anodes are not correctly wired up,(some not even wired at all).
Anode wear can also depends on the water quality where you are, I 'lift out' every two years and by then up here on the North East coast mine is about 50% worn, so I just replace it complete.
Most boats also have two anodes, sometimes externaly but sometimes some engines have special shaped engine anodes fitted.
The most common problem I find is if the boat has a 'pear shaped' anode wired to the stern tube mounting and another wire to say the engine block and the shaft has a flexable coupling, some have no 'jump wire' linking the two shaft coupling flanges either side of the nylon coupling, as if there is no wire then the anode has not got a circuit.
Hope this might help