The process for changing or removing an authenticator hasn’t changed. Nothing about the authenticator system has changed except that the phyiscal keychain is no longer available for purchase.
If their keychain authenticators are still working* and they would like to change to a mobile authenticator they can do so pretty easily:
Once the old authenticator is removed, they can immediately attach the new one.
*If the authenticator is no longer working, this article will still be able to assist them. If they have a SMS number associated with their account they can use that for removal or they can contact customer support if they need assistance.