I observed that a Gen3 VPS can be updated to CentOS 7 by backing up all the server content, then performing a Destroy and Rebuild, then restoring from the backup. My backup was only server content, and was manually performed. But by creating a new account on the rebuilt server, and uploading the content again, it is possible to remain on the Gen3 server.
This information may be useful for someone who wants to retain their existing server, rather than upgrade to the pricier Gen4. I'm not sure when Gen3 servers will be forced to upgrade. I was told, among other things, that GoDaddy's plan was to retire the hardware, although this may be part of the marketing dialog to encourage upgrades.
I also observe that Gen4 runs an operating system that is EOL in 3 years. Is GoDaddy's plan to "encourage" migration to Gen5 servers at that time?
