I've only got a small sample so don't know how wide spread this is.
However, when changing from ar71xxx builds to ath79 the radio number has changed from radio0 to radio1 and on my tplink wr842v(1&3) the ethernet ports have swapped. ie eth0 -> eth1 & eth1 -> eth0
No big deal but if you try and keep an existing config and/or reload one it could have you stumped for awhile.
Have observed this for radio on wr703 and wr842(v1&3). And ethernet change on wr842(v1&3).
Yes, I should have started with a clean slate when updating a snapshot but I have large configurations.
 This isn't change request or bug report, just an observation to try and make others aware.
Before upgrade (no reboot after that) I just change in /etc/config/network/ and it works i my case wr842(v3). Maybe there should be some migration rule
I believe that there is some level of migration scripts in place for the ar71xx-ath79 transition, but I don't know that it's seamless. When I moved to ath79 last year, the hardware references to the radios changed. Not unexpected as the architecture of the kernel and how it defines devices and their drivers changed significantly.
One approach is to move
/etc/config/wireless aside (backing everything up before a major change like this is also highly recommended) and let
wifi generate a new configuration to get the new references. Then it's your choice on how to merge the old with the new.
It's more about being aware of the differences. As you've noted the config changes required are pretty simple to do. Provided you remember to do them