Read the Transition Guide First! Mockups 3 will run side-by-side with previous versions, so no need to uninstall your current version to use it. Here's what's new in 3 and how to import your old BMML files.
The update is FREE. If you have the old version installed, v.3 will automatically pull in the old license. Otherwise just use your old License Information in the Help / Register menu. If you lost it, you can retrieve your License Info here.
Minor-version (3.x) updates in Mockups 3 are free. Simply download the latest version and install it on top of the old one. You won't have to re-register and your old projects will still work.