There's no charge to change your number. (But will that really stop the charges?) When I did it almost a year ago they did charge me the $15+HST twice in the month of change. It's possible they've fixed this bug in their current billing system, e.g. by prorating usage on the two numbers. However even if not, they readily reversed the second charge when I raised this issue with them by Chat.
I suppose you could do the number switch at the end of a billing cycle. But do you do that on the last day (and risk being charged a full month on the new number) or on the first day (and risk being charged for one day on the old number)?