Originally posted by Philip Körte:

I will have him Deckchecked next round. If he manages to do so, there will be no penalty. If he does not, and he calls a Judge on himself for it the instant he presents his Deck, he has commited D/DLP, and I would make his Decklist match his Deck.

Or you could just save the time and effort and do it now (assuming they don't want to simply stick with what they registered).

Well yes, of course, IF he prefers the Gameloss over changing the one card in his Maindeck, we can do that now. Though, especially in the example given, I kind of doubt that, since i'd much rather play a Forest over a Clifftop Retreat than getting a gameloss - and getting a Forest should not be that hard ;).

But yes, if he says he wants to stick to his Maindeck and therefor is ok with the Gameloss, we can fix the Decklist and give him the Gameloss