I'm not sure you should change to the summer tires before you go; especially since you'll be traveling up in the northern tier of the country. You never know when a spring storm will hit and you'd need the traction. Could you lash the summer tires to the roof rack and just cover them with a tarp for the trip?
They're all season tires...should be able to handle a little snow, though they aren't as good as straight winter tires. Putting tires on the roof wouldn't work that well, plus it'd cut down on gas milage...which is one big reason I pretty much scrapped the rooftop cargo carrier idea.