Absolutely. Without question.
And all the more if he spoke ex cathedra.
And it would matter not if it contradicted previous teachings or doctrine. By some mental gymnastics known only to someone deceived by a cult, they could rationalize it and then come to accept it as truth and continue to exclaim that Catholic teaching has never changed over the last 2,000 years.
The process of the development of doctrine in the Church - of the magesterium "growing" in her full understanding of the truth like an acorn growing into a mature oak tree would explain that whole switcheroo nicely! It's always worked in the past.