Just my opinion but with an elderly snake, the last thing you want to do is start throwing them curve balls..."If it ain't broke, don't fix it!"
IF she's a willing eater, I'd sooner give her 2 regular adult mice than one large old breeder mouse, just so you can feel she had a "good" meal. But give them one right after the other, that's important so they don't mess up her digestion & cause a regurge! That's the last thing you want to do. The younger adult mice are higher % protein, lower % fat, more easily digested.
If you start shoving rats at a snake this old that's never eaten them, she might go off feed entirely. You don't know how she'll react, but fear of unknown prey & resulting stress is not unheard of.