I think it's close to getting merged, so okay, done, rebased on top of the DSA PR.
Note that you can't upgrade as usual from a non-DSA build to a DSA build.
As a one time migration you can edit /etc/config/network and change
config device
option name 'br-lan'
option type 'bridge'
list ports 'eth0'
to
config device
option name 'br-lan'
option type 'bridge'
list ports 'lan1'
list ports 'lan2'
list ports 'lan3'
list ports 'lan4'
and then sysupgrade --ignore-minor-compat-version -u openwrt-ipq40xx-generic-avm_fritzbox-7530-squashfs-sysupgrade.bin
.
This worked for me, but you're on your own! Take a backup before doing anything and store it somewhere safe. Better get ready to access the box via wifi because wired might not come up!