Cars using the OBD2 standard ( staring in 95 then mandatory in 96, the dates might move around a bit in CA ) have a up stream ( pre cat, this regulates engine fuel mixture ) and down stream ( after cat, this monitors cat efficiency. )

Wire length / plug is generally the only difference between front to rear for most cars until mid 2000 or so. Around about that time things started to get messy. Some Jeeps used a different type of sensor and Honda started to use a wide band sensor that has more wires and operates completely differently than the old standby.

The big question is, what problem are you trying to solve by changing the sensor?