Tech9
Part of the Furniture
You can try and see how it works for you. Be ready to reset and configure manually, if something is not right. You have a better chance with the same version Asuswrt-Merlin on both routers and AC86U -> AX86U. AX86U Pro is totally different hardware. Once you move to 388 firmware reset may be needed anyway. My advice is to start fresh. My experiment above proves the transfer working, but for basic configuration only and this specific hardware. I wouldn't trust it for VPN configurations and with larger hardware differences. A hidden issue may hit you when you need your router the most.