Hi guys, I'm new here. Just got to know this forum. I'm from Malaysia and I drive a A31 RB20DET.
First of all, S13 & A31 absorbers are compatible, only thing is the coils...which S13 coils are either softer and lower compared to A31. Due to the engine weight, RB20DET is obviously heavier than SR20DET ... you may face bumpiness if you uses S13 absorbers directly, unless you change the coils back from A31.
S13 absorders are slight shorter compared to the stock A31 absorbers. It won't be much problem if you uses S13 height adjustable absorbers. Eventually your A31 will look lower with full S13 absorbers.