It almost feels like you have to break the frame to get the lens out. Once you learn to manhandle the A-frames properly, it's easy (without actually harming the goggles). Just be careful not to scratch your lenses during the switch. I try to grab the lens with the oakley lens cloth so I don't smudge the lens or scratch it.

You can see there are notches on the outside of the lenses shown here. There are little things on the inside of the lens groove that snap into those grooves. In theory you basically pop one lens out of the lens groove and pop the other one in, but in practice it can take some time and can be frustrating to get all the notches lined up and snapped in. Not extremely difficult, but it does take time and attention, enough that you won't want to swap lenses often. But its definitely DIY-able.

Go to the customer service section at Oakley.com. Highlight the lens removal/intsallation bar and choose which glasses you need the directions for. Good to go......

You just pull the frame apart and pop the new ones in.