I have had my IS for over 2 years now and I want to do some things to it. First, I want to lower it a bit. My question is if I put more aggressive springs on my car will I have to change the struts out as well??
It depends on the springs you get. If you get Eibach/L-tuned (1" drop), then probably not for awhile...your stock shocks should be fine.
If you get Tein or H&Rs (1.5-2" drop), then yeah you should probably get some Bilsteins or Koni adjustables. I recommend doing this. Since you've had your car for two years, I'm sure the shocks aren't in prestine condition...and also, a 1.5-2" drop looks perfect.