When you downgrade your package you do not get billed at all with any package with Afrihost. If it is a hosting package you will be dowgraded within 48 hours, often it happens immediately. The new billing will commence beginning of the new month on the package that has been downgraded. However when you upgrade you will be billed pro-rata which is calculated from the day you upgrde to the number of days left in the month. You will get credited pro-rata for the remaining number of days on the current package you are on and get debited the pro-rata for the new package for the remaining number of days in that month. Upgrade will happen immediately.
I hope this answers your questions.