Trick,
You DON'T need to pass any written test. I moved from the US too to Ontario and all I had to do to get the license swapped was get a Driving Certificate from the Motor Vehicle office in the US (very easy to obtain), present it to the License Office in Ontario, fill out the form, show residency and address proof and pay the fee. Make sure you read about all the requirements on Ontario.ca. FYI...Getting Driving Certificate is mandatory to prove that you were indeed a license holder in the US and it is also required to show if you had any traffic violations in the US. Hope it clarifies.
PS: They will keep your US license after issuing Canadian license.
On a side note, make sure you get letter(s) from your insurance company(ies) too that show that you had clean record. This will help you get better insurance rate in Ontario. The insurance rate is ridiculously high in Ontario and if you don't present letter from the US insurance companies, you will be treated as a new a driver and your yearly premium may hit as high as $4000 or even more. Also, Canadian insurance companies will only take your last 3 years driving history into consideration.
Do your research, read online forums and then come prepared to Canada.
Good luck!