The Honda Civic has undergone a change in the model of transmission used from the s20 to the s40. This new model of transmission has significant differences from the older model, but you can upgrade from the older to the newer model even with these changes.
The older s20 transmission is put in Honda Civics built from 1992 to 1995, while the newer s40 transmission is in Honda Civics built from 1996 to 2000.
The older s20 transmission was crafted out of aluminium, while the newer s40 transmission is made of steel. This makes the s40 transmission considerably heavier than the old s20.
The gearbox used in both transmissions is the same, and thanks to this you can use either the s20 or the s40 transmissions in your Honda Civic as long as it is a 1992 to 2000 model.