Light's Hope(vanilla) how do I put on BoP head and shoulder enchants on BoE shoulders

[Bmtppk]

Im talking about things like Falcon's Call or Zandalar Signet of Might or Chromatic Mantle of the Dawn. I tried using the cheap 5 to single resist shoulder enchants as guinea pigs and used the trade window and auction house method "glitch" where you put them into the trade window or AH and start applying the enchants and accept at the last second but 2 things happen:

either

1) Item recieves the enchant and doesnt trade(becomes BoP)
2) It will say "item not found" and the item dissapears but the enchant stays

What am I doing wrong lol