It might not be a clean trade but their are situations where your equity growth in house 1 gives you enough liquidity for a larger down payment on house 2 where your monthly payments might even be lower, especially if it means things like you no longer paying pmi. This is how the property game is played in places with moving real estate markets, I know people who paid more a month for the condo they lived in a few years building equity and seeing gain to afford a down payment on a single family home.