I upgraded a Linksys WRT1900ACS from 19.07 to 21.02.2, but had difficulties and ultimately had to do a hard reset, wiping all of the settings. Now I would like to restore my settings from 19.07, but:
If I upload my settings backup to the Linksys running 21.02.2, I can't talk to the router any more - it won't DHCP, and manually setting an IP address also doesn't let me talk to it.
When I try to downgrade back to 19.07.9 it tells me that the image (openwrt-19.07.9-mvebu-cortexa9-linksys_wrt1900acs-squashfs-sysupgrade.bin) isn't compatible with this router. The displayed sha256sum is correct, so the image downloaded to my computer correctly, and uploaded to the router correctly.
I forced the upgrade, but the router wouldn't talk to me at all; it didn't even activate the Ethernet port. I had to use Recovery Mode to get back to a working partition.
I want to begin with 19.07 because I have a complex switch configuration and I don't want to spend hours learning DSA in order to recreate it.
The settings from 21.02 are not compatible with 19.07 and vice versa. You must configure from scratch. In particular, you have a dsa device (when using 21.02) and 19.07 didn’t use dsa so the settings must be recreated using dsa (on 21.02) or swconfig (on 19.07).
You are running a dual partition router. 19.07 should still exist as it was before upgrading to 21.02. Either use LuCI/System/Advanced Reboot - select Reboot to Alternate Partition or use the 3 cycle off/on using the power switch to get back to 19.07
It will necessarily wipe out all configuration, but yes, the switch configurations are not compatible between swconfig (pre-21.02.x) and DSA (21.02.x and newer).
I said in the original post that I had used recovery mode to get back to a working partition, but because of the number of times that I have tried to solve this problem, both partitions have 21.02 installed.
Again, the most important pieces of the backup (the network configuration) are not compatible between 19.07.x and 21.02.x+ - restoring a 19.07.x based backup on this version would result in an instant (soft-)brick.
"When I try to downgrade back to 19.07.9 it tells me that the image (openwrt-19.07.9-mvebu-cortexa9-linksys_wrt1900acs-squashfs-sysupgrade.bin) isn't compatible with this router. The displayed sha256sum is correct, so the image downloaded to my computer correctly, and uploaded to the router correctly."
Forcing the upgrade appears to brick the router, and I have to switch to the other partition (which is running 21.02).