Switching tools under pressure sounds like a recipe for bad habits. That old guard was basically part of your hand after ten years. A single busy day isn't a real test, it's just a panic move. You probably just needed a fresh blade on the old setup, not to relearn your whole technique with something unfamiliar.
My old boss tried to switch us to a new trimmer mid-summer. Took me three weeks of fighting the thing before I went back to my old one. Muscle memory is real.