It works, so why change?

You can continue to use the same material for 2 years in a row. This is the Renew option.
But keep in mind that the CS4A team works on the curriculum all the time. Sometimes there are minor changes in grammar or spelling, but sometimes the changes are dramatic; like the alignment with ICDL in the 2015 issue and CAPS in the 2016 issue.
That is why you need to upgrade at least once every 3 years.