The difference is you will probably work harder, and sweat more, so dress lighter. Sooner or later, you have to stop and rest, drain, and replenish. You can get rather cold standing around in a fresh coat of sweat. In addition, your feet, instead of flexing and being down on the ground out of the wind are in a fixed position and up in the breeze. There is nothing worse than being 20 miles out in the sticks and realizing your feet are really hurting from the cold.
Warm boots are critically important.
You can work up a sweat to cover your need for warmth, but it is really hard to re-warm your feet. Get off the bike and walk a ways. Better yet, be prepared with warm equipment. Don't ride summer cycling shoes into winter. Check the clothing pages for some recommendations.