You know...I did the same dumb thing about eight years ago (let the CFIIME expire) and have regretted it ever since. A few years ago I called the DE who gave me the CFII, CFIME and ATP rides. She told me that I only needed to take a CFI practical test in a light trainer, say a 152 for example, to re-instate my CFI privileges. I had planned to ask her to give me the checkride but, unfortunately, she passed away from cancer before I could ask her.
Airnik, I'll tell you what...I used to be able to explain and teach compass turns to a monkey when I was a CFI (I had a great time being a CFI by the way). But that was over eleven years ago and many thousands of hours of flying dual FMS equipped jet aircraft with all-glass capabilities. Triple everything. I'll confess to you that without practice I could not do a compass turn to save my bacon let alone TEACH a compass turn at this stage in my career.

Skills not used in aviation are lost fairly quickly. However, like you said, it would simply be a matter of reacquainting myself with the concept (and others not used in a while, like Chandelles) and practicing again - pulling it out of the back of my brain, as you said.
Cheers!
