Reset to defaults is required. You must not attempt to use the old config - it will soft brick your device. Configure from scratch after you finish the upgrade. You can use a backup of the old config as a reference, but don’t restore it.
Should work, at worst there'd always be push-button tftp recovery for this device (so if you rely on the device for any kund of internet access, have everything prepared offline - you shouldn't need it, but better to have it and not need it, than to lack it when you do).
Exactly this is the problem, without this device my internet is very limited. I am planning to setup the similar Zyxel Armor Z2 and do not flash the Netgear before everything workswith the Zyel. Next problem is all the wireless bridges have to be configured new before with the new MAC-address a.s.o.
Can I configure something like <config wifi-iface 'wifinet**3**'> with the data of the Zyxel, so I can easily change? If so, I don't know how to assign / switch to the correct connection.
There is very little (almost no) risk with either the r7800 or the nbg6817, both have very mature push-button tftp recovery mechanisms, so full steam ahead - just keep the necessary files (firmware and tftp client/ tftpd server) handy for the unlikely case of surprises.
That's all you need, tftp-hpa (for the client side, I do like atftp, but that's just personal preference) for the r7800, tftpd-hpa for the nbg6817 - download the OpenWrt 23.05.2 sysupgrade and factory (this one only for the emergency recovery, so just in case) images for the r7800 (respectively later the nbg6817, when you get it) and print out the (to PDF or make a screenshot) push-button tftp recovery information (again, just in case).
This is going to be a straight forward and upgrade, but for cases like this, I always advocate for being prepared (belt and suspenders).
What do you mean? I have no idea at the moment what is the status of the Zyxel, In a worst case, there is no openwrt flashed, but I am very sure it is. It is a backup device, in case the Netgear breaks. At the time when I bought it, it began to be outsold.
He means that you should have a copy of the device-specific tftp recovery advice, like this for R7800. So that you know when to push button during the boot process, do you need a tftp client (like for R7800) or server (like for many tplink routers), does pc need a certain IP address etc.
I will not flash the Netgear until the Zyxel replaced it and works. So when flashing, I will have full internet access. There is also the possibility to use a mobile phone to get something from the internet.
So I will start flashing the Zyxel first, get everything work and then I will flash the Netgear.