Yes, you can upgrade or downgrade your current license to anytime.
When upgrading, the expiration of your annual license would renew from the current date of purchase, and the purchase price would be a prorated price, and will be computed based on the number of unused days in your current plan.
For example, if your initial purchase of Stackable Premium was in January, then you decided to upgrade to a higher plan mid-year on July, the following things would happen:
your license's expiration date would now be on July the next year (originally it was January),
since you still have half a year left from your previous plan, half of the price of your original plan would get deducted from price of the plan you're updating to.