Just dug out my old Linksys EA4500 (on 18.06) and tried to update to current 21.02, but keep getting "The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform." I am downloading the sysupgrade file using the link from the toh page and have downloaded several times just to rule out corrupted download. Do I need to first upgrade to some intermediate version before I can get to 21.02?
You could try installing the 19.07.8 first.
I don’t think a jump from 18.06 to 21.02 is supported on any device since 18.06 is EOL since almost a year.
Remember to not save configs and do not reuse configs.
I think the main issue at play here is the kernel partition needed to be enlarged for these devices at some point (yep - commit 0dbe754e43). Best approach would be to boot into the Linksys firmware (if you still have it) and flash OpenWrt again from there, with the factory image.
Back up your settings first, and reconfigure after a clean install using the backed up configuration as your guide (do not copy them over verbatim).
Hmm, looks like a bit of a PITA to get back to Linksys firmware, and I don't remember what's on the backup partition. I can certainly get back to stock if it's necessary, but I wonder if one of the version upgrades that I skipped will increase the kernel partition as necessary?
You need to install the factory image, so you cannot use sysupgrade. You'd brick your device. Either there's another OpenWrt installation on the backup partition, or Linksys firmware. The wiki explains how the failover works, so you should be able to check it.
I have no idea about how to write a factory image from OpenWrt to the backup partition, maybe the wiki page has it. But I haven't checked.
There is no easy way to handle this change. It is rare and non-trivial, so OpenWrt upgrade mechanisms are not built to handle this. Going 18.06 > 19.07 > 21.02 does not make any difference here. It's 21.02 that needs the bigger kernel partition.