It's simple to prevent involuntary conversion of a reserve line by following CBA procedures. Just volunteer for a conversion and request days off you know they won't honor. And, if they can't honor your days off request, they can't convert you per CBA!
For example, in December, request a voluntary conversion, then request peak week off, 23-26 Dec off, and 30 Dec - 2 Jan off. If they can't honor every single bit of your off day request, they can't convert you.
I know it's counterintuitive to REQUEST a conversion, but if you do, you can prevent it!