Don't let yourself think of tires as an upgrade!
Every 1500 miles or so, you will need new tires!! You can stay with the same kind, or try something different. The fun in upgrading are parts that are you are going to really enjoy-either the feel of or the look

Tires are like bar tape- they are something that you are going to need to replace every few months.

Upgrade things like-wheels, seat post, bars, stem, gruppo, frame!

You can even upgrade helmets and shoes.