Hi,
It's all explained here:
http://www.cisco.com/warp/public/732/licensing.shtml
and here's the relevant part for your situation:
"An individual feature license must be purchased for the specific Cisco IOS Software image for each router on which the image will be used. For example, customers who have purchased a Cisco IOS Software IP image along with their Cisco equipment, and subsequently wish to use Cisco IOS Software IP Plus, must purchase a feature license for Cisco IOS Software IP Plus. The additional feature license will entitle them to use the Cisco IOS Software IP Plus image."
What this means that if you buy a router with IP+ and a smartnet contract, you can download all the new versions of IP+ until your smartnet contract runs out. If you want to move to a new feature set such as crypto, then you need to purchase it. As long as you still have a valid smartnet contract you will then be able to download all new versions of crypto.
HTH - plz rate if useful.
Andrew
