#!/bin/bash
s='ROT: S 3    BL 3900   SPEED 20
    BEN: L 15   
         
ROT: S 2    BLL (DimZ/2+25) BLR (DimZ/2-29) SPEED 20                 
    BEN: L 14-0.5 A 116  AC -1
    
 ROT: S 2   BLR (DimZ/2-29) BLL (DimZ/2-20) SPEED 20                   
     CLA: L 133 A 64    AC -1 
     
ROT: S 1    BLL (DimZ/2-29) BLR (DimZ/2+25) SPEED 20        
    BEN: L 11-0.5  AC -90
    BEN: L 95   AC 1.5 '

sed "s/\( AC \)\([^[:space:]]*\)/\1(\2+PAR)/" <<< "$s"