You can change the subdivision lines in the Piano Roll view.
Go to View, select Customize Control Bar and Display.
Under LCD, select Custom. Tick Signature/Division and any other functions you desire.
You will see the time signature in the Control Bar and right underneath the time signature 4/4, it shows /16 division. Change this to /8 /32 etc and your piano roll will follow the division.
Hope this helps.
Cheers