Why doesn't anyone care to go look for themselves?
Who is eligible to play at the Canada National Championships?
•To participate in the event, you must be a player in good standing and abide by the Pokémon Tournament Rules and all other applicable event rules.
•The Pokémon TCG Canada National Championships is open to legal residents of Canada only. Residency in this case is defined as a person who legally resides in Canada, not necessarily a Canadian citizen.
•Players who are found in violation of the rules of eligibility are subject to suspension from Pokémon Organized Play and loss of prizes, including, but not limited to, invitations and Travel Awards.
The onle bolded is about you. You're no longer a Canadian citizen, but it says that you don't have to be. You are going to be residing (living) in Canada at the time of the Nationals right? If so, then I'm pretty sure that you can participate.
(found this information straight from go-pokemon.com:
Click Here)