When you have a major network change. .reset the Extreme to factory and start over.. they do seem to carry a lot of settings .. and it is hard to make it give them up without full factory reset.
I would not even use the same wireless names.. start over with new ones..
If you run into trouble post again more details... Extreme are connected to the main router by ethernet I guess?? Exactly what mode are they set in? A few screenshots of your setup makes life to troubleshoot much much easier.