Of course. Changing your plan on Merch Locker is simple. Just go to Subscription Settings under My Account, and select the plan you wish to change to.
If you upgrade your plan mid-cycle, then you will be charged a pro-rata basis for the duration of your billing cycle.
If you downgrade your plan, changes will be effective from your next billing cycle.
Note: If you downgrade your plan, you will lose access to any designs uploaded after your initial X designs