How to get sysupgrade package added to Device page

I have Plusnet 2704N V1 installed with Chaos Calmer and want to upgrade. The Device page includes the latest factory image (squashfs) but nothing for the sysupgrade file. If the automated build process is building the factory image why not the upgrade image too?

How do I get the official build process updated to build the sysupgrade file, so that a link can be added to the device page?

Which sysupgrade image? AFAIK there is none.

Thanks T. I guess that's the bit I don't understand: if there have been factory images for v16.x / v17.x and v18.x then how would those users have 'upgraded' to v19? As I read you need the sysupgrade image to upgrade from existing OpenWrt image. How does one get (officially) created - other than personal build of course.