You need to change the plan before it expires... What probably happened is that your plan renewed as soon as you added the funds. If you change it now you'll lose the $15 of your old plan, so you're best off to wait a few weeks and renew the coming round. Or, if you are in a hurry, call Koodo tomorrow and ask them if the can change your plan for you manually :)
To add: There is a $5 fee if you call customer service to request a baseplan change. If the problem is beyond your control, the fee is often waived. There's no fee to change baseplans when accessing your account through self serve online or over the phone. To find out where your $5 went, login to your self serve prepaid account and click on tabs; Billing > Transactions History.