I travelled with my old WP 2 months before it's expiry date, after my BOWP application was submitted online, and had no problems at the border. The officer did bring to my attention that there were 2 months left on the current permit and I replied that I already have new WP application pending (as a proof I printed out the message from "My CIC account" that they send you on the day you apply online), and the officer was very OK with that, and didn't care to see the proof. You should be fine, but just in case print the same page I did. It basically confirms that CIC has received your new WP application. If you applied online CIC will also send you a message to your "MY CIC account" confirming that your BOWP was approved. In my case I received that message 6-7 days before the actual hardcopy showed up in the mailbox. If you get that message on time before your trip, print that out as well. It's not a WP, but a solid proof that you were approved and just haven't received the hardcopy yet.