Using EANx Regulator with Air

[MNBeerdiver]

Just to echo the previous responses, your instructor should cover your questions in class.  I know I had a lot before my EAN class 10 years ago.
The issue of going between air and EAN on the same reg depends on how well the air is filtered.  If I remember correctly, not all shops had air of the same grade that would be necessary for EAN.  The resorts that offer EAN should have their air filtered to the higher standard so you can switch back and forth.  For local diving, if your shop doesn't fill EAN you should ask if they filter to the higher grade of air.  The part in you reg instructions is a CYA on their part.  Many new regs come from the factory EAN ready to 40% and that should be stated in their literature and instructions.