When you upgrade or downgrade mid-month, monthly charges are calculated according to the number of days you have subscribed to each plan. Since you're charged on the same day you have purchased your initial plan, when you upgrade mid-month you're charged a bit higher than the plan's price (the invoice includes a few extra days in your first month). Same thing happens when you downgrade, only the charge is a bit lower. You can find all the information concerning these adjustments on your monthly invoice.
When you cancel your subscription during a monthly cycle, your portal remains active until the end of the billing period. At that point, it is canceled and not renewed.
For example, if you had initially signed up for the Small Plan ($49/month) on the 10th of the month, you would be charged $49/month on the 10th of each month. If, 3 months later, on the 28th of the month, you upgraded to the Basic Plan ($79/month) you would be charged as follows:
Month 1: $49
Month 2: $49
Month 3: $49 (This is the month you upgrade to the Basic plan, but we do not charge you immediately.)
Month 4: $79 + $2.2 (We add a fee calculated for the extra days.)
Month 5: $79
The extra fee above is calculated as follows:
(remaining days in the previous plan * previous plan price) + (number of days in the new plan * new plan price) = (-18/30*$49) + (12/30*$79) = -$29.4 + $31.6 = $2.2