Yes, you can upgrade or downgrade your store subscription at any time. You pay a minimum of one month plus pro-rata through to the end of the eBay billing month when you first sign up. If you are just starting to sell on eBay be aware that you will likely have listing limits placed on your account until you have established feedback.
For more info about store subscriptions
http://sellercentre.ebay.com.au/ebay-for-business/about-store-packages
