Hi Apryl,
Nothing to worry about!
Your software (always the latest version) and license key will always be available for download in your user account. You would find it in your Account management page, "Software" tab. You can lose or misplace a cd-rom, but you can't misplace your downloadable software in your account.
The license key comes with the purchase of LR (included in the price), but is separate from the program in terms of what you need to install. They are separate because the program is continually updated, which you can continue to download for free and upgrade if you wish. But your license key will stay the same. What you do is firstly install LR, then you download and install the license key (as simple as double-clicking on the license key file).
As mentioned, most upgrades will be free (for example, first release was build 1.1.111, but the upgraded version is now 1.2.118), but there may be major upgrades or separate plug in components introduced later which may not be free. This has not been determined yet.
Digg
