Data upgrade from AX 2012 R3 to Dynamics 365 Operations

Hi guys, 

I am in a process of setting up a trial AX 2012 database upgrade for a client, however, finding the pre-ugprade checklist steps confusing. Has anyone completed this process, then please share some light how to kick start this process? Is the data upgrade process is still applicable by preview/invitation only? If so, what's is the process of getting this started? 

I am trying to follow this official MS blog, which says to download a KB to start with, however, it takes me to the my LCS page but links to no particular KB :

https://docs.microsoft.com/en-us/dynamics365/unified-operations/dev-itpro/migration-upgrade/upgrade-overview-2012

Appreciate your help in advance.