Hi Scott,
I also have same scenario in my environment.
According to your suggestion I tried to modify ASM policy which was assigned to VIP from L7 policy Local Traffic>Policies>Policy List.
I had opened policies and also default rule in L7 policy.
In default rule there is traffic mactch crieteria and enable ASM option is there. Also in dropdown list of ASM policy are show.
When I want to change to different ASM policy and I select it. When i went to save it by clicking save tab it is shown as grade out. It will not allow me to save OR do any asm policy change.
I am user with Administrative rights.
This issue I was faced multiple time. When I delete L7 LTM policy then it will allow me. Kindly reply is any way to update this.