If you have developed Features for SharePoint 2007, you’re probably aware of a very serious limitation of Feature Development: upgradability. Deploying version 1 of your customization is no problem, but If you want to deploy a bug fix or additional functionality in version 2, you’re basically on your own. The good news is that in SharePoint 2010, this problem is solved: as a developer you can now build features that are upgradable. The basic concept goes as follows, you built your feature as usual, and optionally you’ve got the opportunity to specify what should happen when the feature gets upgraded from a previous version.
Chris O’Brian has written an excellent series of articles about this topic, so go to his blog to read the details: