I feel reluctant to directly contradict canon in this case by having McGonagall not transform the pig, never mind the havoc it wreaks on all neighboring paragraphs. Do you have a different suggested fix?

"She's McGonagall" is a perfectly acceptable explanation. I had assumed that McGonagall gained the ability to maintain safe transfigurations to living creatures through years of hard work in controlled conditions of the type we gain just a glimpse of when Harry is showing off his new trick. It doesn't seem all that much harder than, say, flying.