When I reach a certain Ploom Club status, is it for life?

No - Here's how it works:

When you earn enough points to move to a higher status level, your status will be upgraded immediately upon earning those points.

In your first year as a Ploom Club member, if you obtain a higher status level, you will keep it until the end of your second year.

After that, your status will updated at the end of each calendar year.

If you have collected enough points within that calendar year to retain a higher status level, it will remain unchanged until the end of the next calendar year. If you have not earned enough points within that calendar year to retain your status, you will be changed to the status level that you have earned enough points to qualify for. 

Then, you can earn points in a variety of ways to increase your balance and regain a higher status level.