"Any woman with a normal pregnancy and no previous history of premature labour may travel up to and including her 36th week on Air Canada, Jazz and Air Canada coded flights operated by Tier 3 carriers."

http://www.aircanada.com/en/travelin...cialneeds.html

Since you will be more than 36 weeks on your flight date, according to AC's policy you would not be able to fly. Seems like a pretty good reason to call in and request the change. If they say no, quote the policy on the website.