2 Why need I explain what is meant by, “for the things that shall be changed”? Every one who is himself “changed,” recognises the meaning of this. Let him who hears this, “for the things that shall be changed,” consider what was before, and what is now. And first let him see the world itself to be changed, lately worshipping idols, now worshipping God; lately serving things that they themselves made, now serving Him by whom they themselves were made. Observe at what time the words, “for the things that shall be changed,” were said.
Already by this time the Pagans that are left are in dread of the “changed” state of things: and those who will not suffer themselves to be “changed” see the churches full; the temples deserted; see crowds here, and there solitude! They marvel at the things so changed; let them read that they were foretold; let them lend their ears to Him who promised it; let them believe Him who fulfils that promise. But each one of us, brethren, also undergoes a change from “the old” to “the new man:” from an infidel to a believer: from a thief to a giver of alms: from an adulterer to a man of chastity; from an evildoer to a doer of good. To us then be sung the words, “for the things that shall be changed;” and so let the description of Him by whom they were changed, begin.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)