Firstly, just because something fits a pattern doesn't make it the correct pronunciation (although I can't see any pattern in your post). As far as I know, "Zee" is the American way of saying 'Z', and "Zed" is the Australian, New Zealand, British, and others way of saying Z.
Why on Earth would you suddenly decide to pronounce it Zed? It doesn't fit with anything. There's no reason for it that I can see. The worst thing is that it completely ruins the ending of the Alphabet Song, which relys on that regularity to rhyme the letters at key points.
Nobody suddenly decided to pronounce it Zed. Actually, the Americans changed their version of the letter Z from the proper English version:
In almost all forms of Commonwealth English, the letter is named zed, reflecting its derivation from the Greek zeta. Other European languages use a similar form, e.g. the French zède, Spanish and Italian zeta, and Dutch zet.
The American English form zee derives from an English late 17th-century dialectal form, now obsolete in England (although it allows the letter to rhyme with "V" in the "Alphabet song" nursery rhyme).
In Shakespeare's King Lear Z is used as an insult. A character is called "Thou whoreson zed! Thou unnecessary letter!" (II.ii), intimating that Z (in Shakespearean English at any rate) is a useless letter, like the person on the receiving end of the insult.
