I'm thinking this is a no worry thing. No need to worry about switching them.
I'm doubting there would be much MPG difference. Or at least not enough to justify the money or time to switch them.
For fun, getcha a tire tread gauge or stop by a tire shop and see just how much tread is used in the trip.
From Click and Clack -
Tom: Sadly, yes, Larry. You have to change to summer or all-season tires during warmer weather.
Ray: Noise isn't so much the issue anymore, as snow tires have gotten a lot quieter. The issue is the rubber compound. The rubber used in winter tires is designed to stay soft and pliable in cold temperatures -- from, say, zero to 40 degrees Fahrenheit. Staying soft allows them to conform to whatever's on the road and give you a better traction footprint in snow and ice.
Tom: But when the weather gets warm, winter tires get too soft. That creates two problems. One is that they wear out very quickly. So if you keep them on all summer, you'll burn through $400 worth of snow tires in no time.
Ray: The second problem is that your handling is compromised in warm weather. Imagine if you need to make an emergency maneuver, and your tires are kind of soft and squishy. You're not going to get the kind of crisp handling you would need.
Tom: So, if you live in a place where you need winter tires for part of the year, you really have to replace them in the spring with something better suited to the warm weather. Or you have to move somewhere that's cold in the summer, too. Like the North Pole. Or San Francisco