No problem! If you buy a 1 or 3 site license and want to upgrade to either the 3 or 10 site license in the future, it's really easy. Simply login to your account by clicking "Login" on LinkWhisper.com.
Once in your account, simply click "Manage License" then click "View Licenses" and "View Upgrades".
This will show you the price to upgrade your account. The upgrade price is prorated based on your current license, so you only pay the difference to upgrade.
Enjoy Link Whisper! :)