Did I miss something, or do we still have very little idea about what the word substitution in Snape's remembrance of the prophecy means?

And Severus Snape drew a breath, and intoned, "FOR THOSE TWO DIFFERENT SPELLETS CANNOT EXIST IN THE SAME VULD."

versus

She couldn't imitate the deep, chilling tone of the original prophecy; and yet somehow that tone seemed to carry all the meaning. "The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies..."

"And the Dark Lord shall mark him as his equal," came Severus's voice, making her jump within her chair. The Potions Master loomed tall by the fireplace. "But he shall have power the Dark Lord knows not... and either must destroy all but a remnant of the other, for those two different spirits cannot exist in the same world."

That last line Severus spoke with so much foreboding that it chilled her bones; it was almost like listening to Sybill Trelawney.