What if a track needs to be registered in both U.S. and Canada?
When registering the ISRC select "Both" in the "Country to register for" field.
PROBLEM: What if on previous registrations I've only been selecting U.S. and I'd prefer to have selected both U.S. and Canada. Is this something that can be fixed?

Resubmit your codes through the registration sites to include either Canada or U.S.. You can either select "Canada" or "U.S." only for the second registration (whichever was left of before), or "Both".