Facebook page - changing web address

Hello,

I manage a page that has an established web address. I don't believe I've ever changed it, and Facebook is clear that I can only change it once. However, I went to change it through the page setting (I'm the creator of the page and admin). I clicked on Change Web Address. All I get is a box that says I already have a user name, but I don't have any way to change the page.

Does anyone know why this is happening? Why am I not able to change the web address? It would be nice if Facebook would just tell me somewhere that I no longer have that ability...........

Thanks!