When you purchase seasons for HD, they go into a pool of available seasons. The pool of seasons is then used for renewing your team.
To manage renewal settings for each world, click the "Manage Renewals" link under the Office menu. From there, you have two different ways to reserve a season for a particular world, ensuring that you won't lose your team.
The first way is by clicking the action link on the right to immediately reserve a season from your available pool for that world. You can also undo a renewal by clicking the action link for a world that already has a season reserved for it.
The second way to renew is to set that world to auto-renew. Then, just before the renewal deadline for that world, the system will automatically reserve a season from your pool.
Note: you must have available seasons in your pool at the renewal deadline for the world or the auto-renew will fail and you'll lose your team. Auto-renew will never purchase new seasons for you with your credits or credit card.