If you change the absorber, better change along the coil spring together, yes kayana rs ultra.. Just wait for the seller la, worth it 😝🐝

Not necessary full rebuilt tho, i have personally seen my OEM/OE absorber serviced they got 1 special tool to insert from top to replenish the gas, if want better they can help also change the hydraulic fluids by tapping one small hole below the absorber and change the fluids. Usually i dont prefer to change aftermarket hydraulic fluids unless they manage to get original fluids from the absorbers manufacturer (Usually is insider ppl sell out) Most aftermarket hydraulic fluids are more thicker in viscosity compare to OEM/OE type thus usually after modify or change will felt very harsh or
"sportier" nature. 

FYI, most of my rides even my dads E90 front OEM/OE absorber also service using the method above. The gas pressure & fluid type really depends on the workshop specialty and experience. If you always take care of the absorber covers, bump stopper usually the absorber center/top seal can last for a long time. Until today most of my rides over 10 years absorber only start to leak (i usually serviced my absorbers on the 5th year after warranty end) and can drag another 5 years before the seal give up and change new absorber.