If you have restored /etc/config/system, you have possibly lost the compatibility version flag and you need to set it again to 1.1.
But only if you are sure that you are now already running the DSA version and are going to flash similar.
Switching from old router swconfig to new router with DSA I only changes config/wireless, config/network
/etc/config/system stayed same -
option ttylogin '0'
option log_size '64'
option urandom_seed '0'
option zonename ''
option timezone ''
option log_proto 'udp'
option conloglevel '8'
option cronloglevel '5'
config timeserver 'ntp'
list server '0.openwrt.pool.ntp.org'
list server '1.openwrt.pool.ntp.org'
list server '2.openwrt.pool.ntp.org'
list server '3.openwrt.pool.ntp.org'
option use_dhcp '0'
you have possibly lost the compatibility version flag and you need to set it again to 1.1.
Decimal change mean incompatible config, like 1.0 - 1.1
Whole number change like 1.0 - 2.0 means incompatible image, a factory install likely needed. (E.g. when wrt32x kernel area size changed, sysupgrade is not powerful enough)
The value can be set for a router/target, when a change like DSA is made, highlighting to you that the sysupgrade needs care.
But as it is just a value in system config file, the user may accidentally overwrite/remove it (like you?) in connection of old backup restore, settings copy or similar.