Actually, thats not entirely correct.
"Y-Head Cannon" can only be equipped to an "X-Head Cannon". If it is equiped to any other monster, it would be destroyed.
This is consistant with the following ruling:
Relinquished FAQ:
[Re: Collected Power] Monster Cards equipped to "Relinquished" or "Thousand-Eyes Restrict" can be moved to another "Relinquished or "Thousand-Eyes Restrict" but if either of these monsters is already equipped with a Monster Card, the new one is destroyed. If these moved to any other monster, the cards are destroyed.
Therefore, while you can equip "Y-Head Cannon" to an "Elemental Hero Prisma" or "Phantom of Chaos" while its name is treated as "X-Head Cannon", the moment the name of these monsters reverts to its original name, "Y-Head Cannon" is destroyed, as it is now equipped to an illegal target.
Hope this helps
No one's been able to take me seriously... But that will change when I beat you at a children's card game.