Subscription Upgrades

I am currently developing a web app that has a few different price points depending on how many people will be using it.

For example

Personal - meaning one person is free
Small Business meaning 5 or less people is $30 a year
Medium Business is 6-15 people and is $60 a year.

That is all fine and dandy and easily created. The problem I just can't quite figure out is allowing people to upgrade at anytime. So lets say a small business grows to become a medium business. They would like those new people to be able to log into their group as well. How do we charge them here.

They payed $30 for the year on Jan 1st it is now April 1st and want to upgrade to the $60 package. How do we go about getting this to work correctly for future payments. If they pay the difference of $30 and we start a new year from that point which would end on April 1st the following year they effectively have payed $60 for 15 months instead of the 12 month time span.

The other way would be charging $30 more and keep the original Jan 1st date but then they will be paying $30 for a year and $30 for 9 months which screws them over.

Is the best way to go about this prorating the cost of the upgrade. So instead of charging them $30 for the upgrade we would charge them $22.50 and keep their renewal date at Jan 1st. This effectively has them get what they pay for no more or no less so it works out good.

I was mainly wondering how other people do this. Any help please.

Most businesses prorate the cost.