You can upgrade at any moment during a month. Your billing date will be reset on the day you upgrade for a bigger plan. All pictures took on previous plan will count in the new. This means you will pay less per picture by upgrading.
Here's an example:
Let's say you are subscribe in the starter plan ($2.50/picture, $20/month) and your billing date is the 1st of December.
On December 10th, you have shoot the 8 pictures included in your actual plan... BUT you still have 102 pictures to shoot to be ready for Christmas...
What should you do?
Upgrade for the Executive plan save Money!
The Executive plan includes 110 pictures/month for $200/month ($1.82/picture).
When you upgrade, we reset your billing date from December 1st to December 10th.
Instead of paying $2.50/picture for the 8 pictures you took on the Starter plan. Your 8 pictures will be charged at $1.82/picture as if you would be on the Executive Plan from the beginning. They will count on the 110 pictures included monthly on the Executive Plan!
The end results: all your product pictures are ready for Christmas and you pay a ridiculous $1.82/picture !