Let's say that whenever Vista comes out, you decide to go the economical route & purchase the "Home" version. A couple of months down the road, you then find out that it doesn't have as much functionality as you'd hoped for, & want to get the "Premium" or "Ultimate" versions. At least this time around, MS won't make you buy the full version; you'll be able to simply upgrade.

"Microsoft Corp. on Wednesday confirmed that a feature called Windows Anytime Upgrade was in Vista, the next major update of the operating system, but declined to discuss details."