I’m not speaking to any changes in liturgy introduced by individuals who are not following the dictates of the Holy See. I speak only to what “the Church” can do.
Yes, the Pope has the right to modify any liturgy of the Church, including the sacraments, and there is no one with the authority to correct his decisions. (This makes the third time today I’m quoting Mediator Dei on this point. My apologies to those who wish I would not so belabor it.)
According to Pope Pius X, in exerpts from Mediator Dei: (boldface and underline: mine)
"… Our predecessor Sixtus V of immortal memory established the Sacred Congregation of Rites, charged with the defense of the legitimate rites of the Church and with the prohibition of any spurious innovation. This body fulfills even today the official function of supervision and legislation with regard to all matters touching the sacred liturgy.
…It follows from this that the Sovereign Pontiff alone enjoys the right to recognize and establish any practice touching the worship of God, to introduce and approve new rites, as also to modify those he judges to require modification…
**…the sacred liturgy, as We have said, is entirely subject **to the discretion and approval of the Holy See…
…The liturgy of the early ages is most certainly worthy of all veneration. But ancient usage must not be esteemed more suitable and proper, either in its own right or in its significance for later times and new situations, on the simple ground that it carries the savor and aroma of antiquity. The more recent liturgical rites likewise deserve reverence and respect. They, too, owe their inspiration to the Holy Spirit, who assists the Church in every age even to the consummation of the world. They are equally the resources used by the majestic Spouse of Jesus Christ to promote and procure the sanctity of man.
…Clearly no sincere Catholic can refuse to accept the formulation of Christian doctrine more recently elaborated and proclaimed as dogmas by the Church, under the inspiration and guidance of the Holy Spirit with abundant fruit for souls, because it pleases him to hark back to the old formulas. No more can any Catholic in his right senses repudiate existing legislation of the Church to revert to prescriptions based on the earliest sources of canon law. Just as obviously unwise and mistaken is the zeal of one who in matters liturgical would go back to the rites and usage of antiquity, discarding the new patterns introduced by disposition of divine Providence to meet the changes of circumstances and situation.
…Only the Sovereign Pontiff…and with him, in obedience to the Apostolic See, the bishops "whom the Holy Ghost has placed… have the right and the duty to govern the Christian people. Consequently, Venerable Brethren, whenever you assert your authority . – even on occasion with wholesome severity – you are not merely acquitting yourselves of your duty; you are defending the very will of the Founder of the Church