Note: Switch to the opposite partition that you want stock installed on, flash stock, then switch to the partition stock was installed on (example: you switched to p2 and flashed stock, so stock is now on p1), reboot.
Jim, I tried your solution but tfpt would not connect with the router even though ssh was connecting fine. The issue is solved now, see the other post.
If the wiki TFTP flash is to be believed, a serial connection is required for tftp. I have only done it once and never bothered to check if there were any kind of reasonable default IPs. But in practice this is seldom actually required on the wrtpac devices, they tend towards the fail-safe end of the spectrum.
The bigger question is why the current stable did not let you sysupgrade -F to OEM as would be expected.
Say I have partition 1 set as OpenWRT and partition 2 has stock firmware installed.
If I just want to update the OpenWRT to a newer version by upgrading within luci, will this overwrite the stock partition and switch that one to the active one? Is it possible just to update the current partition?
Not easily.
You might use the "advanced-reboot" LuCI app to switch to use the other partition with Linksys firmware and use that to flash a new Openwrt to this currently running partition, and then restore OpenWrt settings (which will be destroyed).
It might be also possible to trick the system to flash the current one, but that requires some u-boot knowledge. If you familiarize yourselves with the dual-boot mechanics, you might figure that out and test if that works.
In general, there is no special reason to keep the OEM in the other partition. You can alwats re-flash the OEN firmware, if you want to revert back to the OEM firmware later.
Thanks - answered my question. It's just that WiFi performance is significantly better on stock and would be useful to keep it just in case I needed higher speeds but then that means I can't ever update OpenWRT so it's not really a suitable option.