Heh, I did that a couple of weeks ago.

Commute in this morning was fine. Tried the Marathon tires for the first time on my commuter (the folder). The switch from faster tires didn't slow me down too much but it did add some peace of mind for the ride home in the dark (more junk on the MUP from a day of careless / non-caring users ==> faster tires getting chewed-up).