Knowledge is king...

Since I'm a nerd and I feel the need to know the most about shit that I can, I've been doing research on the redesign of the Sportster for 2004 (since I now own one). Not only was the rubber mounted motor essentially changed to the Buell design, which in turn made them change the frame design altogether, they also changed the transmission. In earlier models, the transmission was removable through the primary with a trap door much like its big twin brethren (of course the big twin trap door face the right side since the drive side is opposite of a Sportster). In 2004 though, they opted to make it impossible to access the gear set as a cassette like earlier models and now you have to split the cases to do ANYTHING to the transmission. Now, this isn't a huge deal, but if you had intentions of doing anything aftermarket such as changing to a Baker reverse pattern drum or an XL6 gear set, then the 04-up will not accomodate those changes.

Here is a good picture of the inside of the left case that I took from XLForum.net.