I would like to install openWRT 21.02 in my WNDR4300 v1. I already read here, but I get a warning during the upgraded (I tried to upload ath79 21.02 on ar71xx on 19.07.8). I cannot move first to ath79 on 19.07 since it is not available.
Can I proceed? Have I to reconfigure it from scratch?
This message is normal, although alarming. It is due to the change of target and version.
I also had similar issues with the 3700v4 which is almost identical to the 4300.
I would recommend using a tftp install, or returning back to stock firmware, and than flash a factory image.
Both these ways work. Than configure from scratch.
No. The kernel partition size was increased. Performing forced sysupgrade on this device will basically put it in recovery mode. Install from scratch, using recovery (the tftp based method) and factory image.
Restoring an ar71xx based config to an ath79 device is not safe, there are differences in multiple places (wireless paths, GPIO/ LED configurations and more). Only use the old configuration as reference for your new one.