Noname is basically correct - for normal shocks you drain out the old oil and replace ONLY TO THE FILL LINE with new.  While you are at it, you may want to replace the internal springs or change their dampening by removing or adding shims.  Gas charged shocks are typically nitrogen filled and are none servicable by home mechanics.  Most gas filled shocks are expensive, but the upside is they can be rebuilt by the original company - e.g. Ohlins.  If your bike has gas filled shocks, you will need to replace or rebuild them.  Shocks for the most part are one item I wouldn't buy from salvage as ALL shocks slowly become less effective over time, either though the oil becoming dirty, leaks, etc.  Unless my originals were bent badly, then I might consider salvage and an immediate rebuild.