How to adopt my own old handle?

The organization I work for has been using a handle for a few years now that doesn’t match our facebook handle or website url. The reason being that over the years we just lost the login and email access for our account with the correct handle. As of today, I FINALLY managed to get access to this old account. My question is how do I take the handle from our old account and apply it to our active account? I was trying to look for an answer online and it seems that if I change our old account’s username the handle will become available, but it might not become fully available for a while? Is a while a couple of hours? Days? Months?