ccool, do not stress over this. It is very common for stamps to be available to individuals before the day of issue. With all the deals that
Canada Post cuts with corporations to issue special stamp souvenirs, covers, etc., before the day of issue, it is normal for specific people to have access to them before the official first day of issue. I have personally had a couple of releases weeks before the OFD. While it my be interesting to get a few cancelled just for fun, these do not amount to much value because they are not very rare, extremely philatelic in nature, and fairly easy to fake given the crazy array of cancels now available.
On the other hand, I would not worry that your post office will object to cancelling these stamps for you. It is my experience that they generally do not know or care about such matters.