I am using an older version of lightroom. I want to upgrade to Lightroom 3. In order to keep all treatments of previous files, do I need to download the upgrade using the same serial number or can I purchase the new version (with a new serial no.)? If that is possible, is there anything I need to know in order to keep all stored info?
You'll get a new serial number when you upgrade. You can then open your old library in your shiny new LR3 (which will create a separate, updated copy of it) or follow the somewhat more convoluted route which Jeff Schewe (who should know) recommends in the LR3 videos.
Either way, all your existing adjustments will be preserved although some may well need tweaking if you update the shots to process 2010.