The past few days I've seen a couple of WiFi configs posted (here and on reddit) from /etc/config/wireless that seem to have radio0 configured as 2.4 Ghz, and radio1 as 5 Ghz.
I've always thought the default was the other way around...and I think it has been on LEDE/OpenWRT since I started using version 17.
I understand someone could physically edit the wireless files and change the radio references, but that seems like a lot of work for no gain, AFAICT.
You cannot swap them at will. It all comes down to the device you're working with. Which radio sits first on the PCI-e bus etc. The wireless config will have the radio type set either way, if you want to be completely sure what's what you can take a look at the DTS. There the radios are defined with their supported frequency ranges.