Upgrading Linksys WRT1900ac v1 to 21.02-rc1

I have been a satisfied user of OpenWRT since 2015, with a Linksys WRT1900AC v1 router. Upon trying to upgrade to 21.02-rc1 from 19.07.5, with

sysupgrade -v openwrt-21.02.0-rc1-mvebu-cortexa9-linksys_wrt1900ac-v1-squashfs-sysupgrade.bin

I get the following error:

Device linksys,mamba not supported by this image                                                                        
Supported devices: linksys,wrt1900ac-v1 armada-xp-linksys-mamba linksys,mamba - Image version mismatch: image 2.0, devic
e 1.0. Please wipe config during upgrade (force required) or reinstall. Reason: Partition design has changed compared to
 older versions (up to 19.07) due to kernel size restrictions. Upgrade via sysupgrade mechanism is not possible, so new 
installation via factory style image is required.                                                                       
Image check failed.

What is my least risky upgrade procedure - preferably via the OpenWRT command line? Or should I wait for a later release?

Runs fine on a mamba, just follow the instructions. Be aware that besides the larger kernel reservation space, this version has also moved from swconfig to DSA, and that 21.x does not have any GUI support for setting up vlans on DSA (currently); this does exist in master though.

sysupgrade -F -n factory.img

Thank you. All worked fine until I used sysupgrade -r under 21.02 to restore the 19.07.5 configuration. After a reboot, there was no network (including Ethernet) and the router's fans ran loudly and continuously. I managed to return to 19.07.5 by booting the alternate partition. I then ran the sysupgrade again and, this time, reconfigured manually.

You can't restore the 19.07.5 config to 21.x. The DSA changes prevent that. Install 21.02 and reconfigure everything from scratch.