I recently completed the FI course (May 2018) having already been a CRI. I found no way of avoiding any of the 30 hours flying element of the FI course but the school decided that they would give me full credit for the 30 hours (I seem to recall) ground instruction that I'd received as part of the CRI course.
Although initially disappointed I actually found 30 hours flying just about right, we did the course in about 20-22 hours and then revised the exercises which really helped to drive them home.
I'd suggest you make sure you explicitely tell CAA that you want to keep the CRI when you get the FI rating added to your licence, I'm not quite sure why but it just feels better to keep everything so as to have the most options in the future.