The changes in the words of the form in the Latin original, although certainly illicit and unprecedented in the history of the Church, do not alter the substance of its meaning, and consequently do not invalidate the Mass.
The validity of the reformed rite of Mass, as issued in Latin by Paul VI in 1969, must be judged according to the same criteria as the validity of the other sacraments; namely matter, form and intention. The defective theology and meaning of the rites, eliminating as they do every reference to the principal propitiatory end of sacrifice, do not necessarily invalidate the Mass. The intention of doing what the Church does, even if the priest understands it imperfectly, is sufficient for validity. With respect to the matter, pure wheaten bread and true wine from grapes are what is required for validity. The changes in the words of the form in the Latin original, although certainly illicit and unprecedented in the history of the Church, do not alter the substance of its meaning, and consequently do not invalidate the Mass.
However, we all know that such a New Mass celebrated in Latin is an oddity, doomed to extinction by the very fact of the reform. The validity of the New Masses that are actually celebrated in today’s parishes more than 30 years later is a quite different question. Additives to the host sometimes invalidate the matter. The change in the translation from the words of Our Lord, "for many" to the ecumenically acceptable "for all" throws at least some doubt on the validity of the form.
In other words some Novus Ordo Masses may be invalid, but not necessarily all of them. Why take the risk?
It does more than say "why take the risk?" It goes on to declare that it is illicit to take that risk, and then leaps to the conclusion that since it is illicit to take that risk, it is illicit to attend New Order mass. But there are other ways to avoid taking the risk: Priests who confect invalid masses do so purposely rebelling against the Church. Such rebellion is easy to recognize if you educate yourself. The things which invalidate a mass have been posted here several time, due to the wonderful work of Freepers, but those are external signs; it is true that the priest must also have the proper disposition.
The following are irregularities that do not necessarily invalidate a mass, but which suggest that the mass is offered without proper disposition:
* Use of extraordinary ministers while there is an idle concelebrant.
* Use of sacred vessels which are not obviously noble. (Some glazed pottery, for instance, does not necessarily desecrate the Eucharist as unglazed pottery, but why would the priest tolerate the uncertainty?)
* Ad-libbing rites.
* Laity in the sanctuary.
* Disregard for the GIRM.