Complex sentence: Field service could convert a 615 to a 635 or 625 or vice versa in a couple of hours if necessary; other than those few wires, the 615,625 and 635 were identical.
Split into simpler sentences: Field service could convert a 615 to a 635 or 625 or vice versa in a couple of hours if necessary. Other than those few wires, the 615,625 and 635 were identical.