Button changing its text & action. Good or terrible?

After the user Registers for an event (he goes to cart and pays, etc.) the next time he visits the page, the event for which he registered now shows a less emphasized Unregister button, which does the exact opposite of what it did until the event was purchased.

Is it a good practice to have the same button change it's function or is it bad and confusing?

Update:
If it helps anyone, I went with this layout: