Do you use the eBay Sell Your Item form?
Just change the currency from US to CDN for you next several listings.
Eventually it will learn to do this automatically.
For your current listings, you will have to close them all.
Don't panic.
They will go to your Unsold List for 90 days.
You can then use Sell Similar to relist them and in the process make any changes that you want, including currency.
I don't think this can be done in bulk.
I'm not convinced that the change will be beneficial. However, the eBay notice about it did say that changing to USD was not useful in our primary category (media/ books).