You can change your rate plan twice a month. You do this by changing your settings in the customer zone. Note: if you switch to a more expensive rate plan, it is immediately activated. If you switch to a lower rate plan, it is only activated at the end of your current period.
For example: your subscription starts on 19/10. On 25/10, I upgrade: I have two bundles now. One that expires on 19/11 and another that expires on 25/11 and immediately renews.
My subscription starts on 19/10. On 25/10 I downgrade: my bundle expires on 25/11 and I immediately receive a new, lower-rate bundle.