Most definitely you don't want to switch the back as each tooth you drop from the rear sprocket results in almost a full 3 tooth change to the front sprocket. You go UP in size to make gearing easier, which means you are adding weight to the bike which is also counter-productive.

A big issue may be that the hub you have likely has a one piece sprocket/driver so you would need to get (find) the driver that allows interchangable sprockets on it. The 13 tooth Odyssey driver I am pretty darn sure is one piece. This means you will end up paying 30 bucks (US) or much more likely more in your location, for a rear sprocket change, then you must disassemble the entire rear hub to make the change. A formidible task.