Facebook allows to change the Facebook Web Address once if you
enter in your Facebook page >
Page info >
Facebook Web Address.
Facebook does not allow to do it a second time. I have found a "trick" to do it, though it does not seem safe. The trick is merging it to a new page: http://www.customgrowthgroup.com/how-to-change-your-facebook-page-vanity-url-for-the-2nd-3rd-or-x-time/
Do you know if it is safe? Do you know any other way?