@Shaz when I first heard about the cameras I just went into the service app and requested service to upgrade the cameras even though the option wasn't there. They then contacted me and said there was no upgrade available at the time but I think you get put on a list by doing that and then when it was available you got notified.
I've attached some screenshots of what you should select and then in the "describe concern" of the service just put " FSD camera upgrades for legacy X".. as by creating a Tesla service request, you'll get a reply back from a Tesla Tech within the app chat within a day and you can ask them to put you on the list or what the expected ETA is.
The tech who did my car said he was going to another X right after my appointment so I'm not the only one.