Automatically change to 2G when connected to WiFi

[mj0]

Is it somehow possible to have my phone (Lumia 635, Windows 8.1 Cyan) automatically change the mobile network settings to 2G instead of 3G/4G as soon as I'm connected to WiFi? It'd do wonders to battery life if the phone would simply switch to 2G, I really don't need HD Voice (hasn't ever really worked anyway) and mobile data is channeled through WiFi thus a 2G connection would easily suffice in this case. As soon as I lose my WiFi connection the phone should automatically switch back to 3G/4G.

Is that possible?