Tires tires tires

I have a 2008 2720 TM that I just purchased. I don't know how long the tires have been on the unit, so considering replacements is high on my to do list.

I am also considering upgrading wheels. The unit is currently equipped with 14" wheels and tires. What modifications are necessary to enable 15" wheels and tires? A lift kit isn't really an option because the trailer just clears the top of my garage door.