Doing an immediate plan change sometimes the auto-pay is not credited by the system and is only applied the following 30 day payment.
Link below retired Oracle computer geek mentions this.
Not sure if moderators can help crediting you with this but you can ask them.
Did you lower your plan as you plan to leave Canada for a while but plan on returning at some point? If that's the case then you should remove auto-pay from your account as the system would continue to charge you while you're away. It's better to remove auto-pay and at next payment due date your account will go to suspended status. 85 days in suspended status set a reminder to log into your account to make a payment and reactivate your account as at 90 days your account is deleted and you loose your number.
If you plan on not returning to Canada at all. Delete your credit card from your account. To do so log into your account click "Payment" then bottom left "Manage my card" to remove it.
To remove auto-pay you click payment then "Manage auto-pay" and you'll have the option to stop it. If you're planning to return to Canada at some point. It's cheaper letting the account be suspended for 85 days (you save 2 payments) then letting auto-pay charge you each $30 days. You can also disable auto-pay by dialing 611
