Yes, because I plan to convert back my RT-AC86U to run ASUSWRT-Merlins firmware, I have not modified any system files etc. etc.
So I Factory Reset RT-AC86U, then put it into recovery mode and flashed using ASUS Recovery tool with an old (first release) of the GT-AC2900 Firmware.
As long as the router is not then factory reset, it pick ups latest GT-AC1900 2900 firmware when it goes online after manual configuration.
At one point (I was testing) I flashed the RT-AC86U with GT-AC2900 firmware and then performed a factory reset, after the reboot and configuring the RT-AC86U, it notified me there was an update, which I applied and it resorted back to the RT-AC86U ASUSWRT UI.
Early days yet, but looks good and works well, only have noticed that the option for Game Port Prioritiation > (level 1) > there is not an option to Select “Dedicated gaming port that prioritizes network traffic to connected devices”. This maybe due to Firmware not being as good as it should be ?
But for testing purposes, very happy I am 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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