I really don't think it's that noteworthy or remarkable that Mexico or any other country is indicating that they want to work with the new president and are willing to change their previous positions to that end. Or, to put it another way, I don't think Mexico changing its position is uniquely conditional on Trump being the president, as if they'd be willing to blow off any other president, but Trump is just such an impressive and fearsome man that Mexico doesn't dare disobey him. America is always a force to be reckoned with in international politics, and the president is always somebody who needs to be taken seriously. There's only so much that world leaders can push back on the president, let alone openly defy him on.