Am currently running OpenWrt 21.02.0 on D-Link DIR-835A1 ( https://openwrt.org/toh/d-link/dir-835_a1?s[]=dlink&s[]=835 ) and am planning to upgrade it to either v23.05 or v24.10. Last time I upgraded from v19 to v21, I just flashed the v21 sysupgrade image from the OpenWrt web interface and everything worked almost without issues (just had to reinstall some packages and that was it).
With reference to the above:
Do you think flashing the sysupgrade for v23/24 would work and, if not, what the potential problems might be?
What would be the proper way to upgrade the router to v23/24?
The hardware is kind old, so any reason to go with v23 over v24?
If architecture is ar7xxx (not ath79) then you absolutely need to reset config and re-type configuration back from backup.
Otherwise 21->22->23->24 , must retype if you see red alert asking to force update at any step.