What G-code commands tell the printer to change nozzle on a dual extruder printer?
No, M108 does not do that. You are looking for T#, where # is the tool position you want:

T1 ; switch to tool position 1 

T3 ; switch to tool position 3 


This tells the processor to send all heating, cooling and flow commands to this tool until another tool change is specified, and invokes the X/Y(/Z) offset for the new tool position.

See http://reprap.org/wiki/G-code#T:_Select_Tool