Here how it works in reality, tested and verified with NVRAM comparison, GUI settings comparison and functionality:
The same router older to newer firmware -
all good.
The same router newer to older firmware - may need fixing here and there,
not guaranteed.
If the new firmware has password encryption, but the older one doesn't for example -
factory reset, no way to log into the GUI.
RT-AC68U -> RT-AC86U -> RT-AX86U logical upgrade path -
all good.
Notice
the conversion of DHCP reservations, custom device names, custom icons between ARMv7 and NHD - it was
done automatically.
RT-AX58U -> RT-AX88U (both on 388 firmware) -
all good.
RT-AX88U -> RT-AX86U (both on 388 firmware) -
all good.
Host with extra features to target without - changes to NVRAM transferred,
but ignored.
Host with fewer features to target with extras - extra features remain at
default values.
Dual-band host to tri-band target - the extra radio will be on default, existing radio if incompatible settings will be
set to Auto.
Tested with RT-AC68U -> RT-AC5300
5GHz-1 radio (ch.149 on RT-AC68U) was on
Auto channel, 5GHz-2 radio (non-existent on RT-AC68U) was on all
Default.
Two reasons I did not recommend
@fffddd doing it:
- Asus save/restore is
unaware of Asuswrt-Merlin changes on top (more on this below)
- I don't have newer BCM4912 hardware and
haven't tested it myself
Asuswrt to Asuswrt-Merlin -
basics work, whatever is different (like VPN configurations, script changes, etc.)
doesn't (as expected).
Asuswrt-Merlin to Asuswrt -
cleaned properly configuration to the features available in Asuswrt works, otherwise needs fixing. Forgotten CakeQoS will invoke Unknown QoS type message, has to be changed/fixed to Asuswrt available options by enable/disable. Forgotten VPN configurations will just hang there unused/unrecognized, may be a problem for routers with low NVRAM. Any script changes have to be reverted to defaults before save/restore, but unfortunately I found some scripts don't clean well after uninstallation. More issues with scripts, but not subject of this conversation.
Now you know why
Asus doesn't have hardware and version check. Their save/restore simply works most of the time on incremental upgrades or replacements. Notice the FAQ says "your Asus router" and not "the same hardware and the same firmware". Third party firmware users already accepted the "use at your own risk" condition and have to follow the developers recommendations. If they don't - their problem.
Some people like to repeat what someone else says without testing. This is how
forum mythology was born.