My Evo 9 front pairs of KYB struts has blown, and I am replacing it with Bilstein HD Struts. Does it matter if I am using Bilstein HD Struts in the front and keeping the KYB in the rear ? ( since the rear struts are still good ) will it cause any future problem to the car ?

 

Kinda like mixing tires and the difference you'll feel front/back. Mixed brands + mixed compression/rebound rates + mixed performance class = mixed handling from front to rear. Driveable....yes. Potentially unpredictable under certain conditions....yes.