You could maybe keep some in an airtight container somewhere to use as a substitute for critical care in an emergency? Mashed baby pellets mixed with warm water would be a decent stand in until you can get critical care.

Also I would try weaning the bun on to it gently; use a mix of baby and adult food until you’re out of the baby food. It’ll help the bun adjust, and I don’t think a little extra time on baby food will do much harm unless there’s a weight issue or it’s affecting hay intake?

I’d just use it all up in the transition to adult food.