You aren't intended to edit the files in /etc/config directly, though there is obviously nothing preventing you from doing so.
However, using UCI to modify your configuration is preferred as it could avoid syntax errors that may be difficult to diagnose.
If I set my WAN interface from pppoe to dhcp using UCI, the command is as follows:
uci set network.wan.proto=dhcp
If you cat /etc/config/network, you'll notice that the file itself still says pppoe. However:
uci get network.wan.proto
Any processes which read the value will receive "dhcp" and not "pppoe". If you want this change to be permanent, you call "uci commit" and now the change is written to the file.
You should pretty much never need to start a dhcp client manually. Network setup, among most other router functions, are handled in init scripts located in /etc/init.d
After changing your network configuration, you'll want to either:
Please read the documentation. It will save you lots of time and trouble.