Mails from CIC state "DO NOT FORWARD" on them.
For something as important as your test/oath letter (or ANY letter from CIC, for that matter), I would definitely NOT risk it at all.
My advice would be to update your address - and even after you've done so, to keep an eye out on your old mail for a while at least.
PS- Something else that i just thought of - if the postman cannot deliver your letter (do not forward) it would probably go back to CIC. I'm assuming this could trigger something with CIC regarding your(/anyone in question) file.