Obviously Pius V did not prohibit himself or his successors from authorizing changes to the Missal. He only prohibited the making of changes without proper authority.
I thought it was just the Canon that was the unchangeable part. I am certain that my mother's (+ rip) old St. Joseph Daily Missal stated that the unchangeable part of the Mass is the Canon. I've had to put her missal up because it is so well worn that the pages are literally disappearing. Maybe someone else can provide a source.
Here's some food for thought.... The Novus Ordo is not a change to the Mass - it is an entirely new creation and completely separated from the Traditional Mass. The NO cannot be compared to previous changes in the Missal.
"After the Council… in place of the liturgy as the fruit of organic development came fabricated liturgy. We abandoned the organic, living process of growth and development over centuries, and replaced it, as in a manufacturing process, with a fabrication, a banal on-the-spot product”. - Cardinal RatzingerI found that earlier today in a Latin Mass article. LINK
Edit to add another link with the above quote.