I had a lot of connectivity issues with upgrading my wifi. What worked for me is a combination of things I found in this forum.
First, borrow your Guest network. Change the name to the same as your previous network with the same password. Some of your devices will need to be on the active active to disconnect them to add them to your new wifi (Feit was my particular challenge)
When connecting devices the best mode (if available) was the AP mode.
Nest Protect devices had to be rescanned - so annoying since it’s all Nest.
Once everything is connected to the new Wifi you can change your Guest wifi name and password.