That is the correct practice.

Because you prepay for your plan on prepaid you don’t get reimbursed for any days you didn’t use of the old one.

Theres a little note about this right under the “change to this plan” button:

My bad, but my own fault for not reading at all, lol. Thanks so much for the quick and informative reply.

Around renewal time, the prepaid site can go a little wonky. Might give you a message saying your plan has expired while it’s busy digesting a recent payment.

Best practice: go on auto-top up and avoid the site entirely for a few days around renewal. No double-charge or lapse in service. If you need to make a baseplan change, wait till your current “month”  (really 30 calendar days) is just about expired.

If you need to snooze your service for a while, you can do so by simply not paying at renewal time. Baseplan will expire, no service, but boosters remain intact for further use if you re-activate baseplan within 90 days. After that, account is closed, SIM is cancelled permanently and number is lost.