In case you missed it in Amazon's terms, the free upgrade is temporary. It expires in a year, at which point you can start paying or be bumped back down to the free level.
...you will be automatically eligible for the 20 GB plan for one year from
the date of your MP3 album purchase. Unless you set your account to auto-renew
to a paid plan, the 20 GB plan will revert to a free plan one year from the
date of your MP3 album purchase.