The discussion on the RE220v1 may be helpful.
On that one the process was.
Solder serial connector.
Interrupt boot and install the re200-v3 initramfs firmware
(after everything tested ok), use sysupgrade to install the re200-v3 sysupgrade firmware
[warning may brick device]
test it some more.
do a build with the name changed from re200-v3 to re220-v1 in a few different places
There is a program called mktplinkfw.c that is part of the build process and it 'fixes' the firmware checksum.