Try this link:
http://traxxas.com/v2/powerup/
SELECT: the model or part you want to Power-Up!
CHOOSE: your Power-Up and any additional parts!
CHECKOUT: using this site!
CONFIRM: your Power-Up!
SEND: the Power-Up form and your original Traxxas engine, power system, or radio system to Traxxas. You’ll need to keep some parts such as exhaust headers, starters, and pinion gears. (All the details will be explained after you select your Power Up items)
NOTE: The Traxxas Power-Up program is only for U.S. customers.
It doesn't matter where the engine came from, you can trade in any engine for an equal or higher performance engine. You can trade that 2.5r for another 2.5r for $85 or trade up to a 3.3 for $110.
Another option "if your confident enough to do so" would be to just rebuild the 3.3 mill yourself. This is a much cheaper option and is a great learning experience.
Most any hobby shop that carries Traxxas products participates in this program.