What do you want to upgrade on a D8? Saddles are always upgrade able. Gearing yes but it is more complex than upgrading a normal bike since the derailleur is different. I think but haven't done this, you can get a modified derailleur hangar and use a standard derailleur, so wider cassette range can be used. Don't know about bars. I have thought about doing some upgrades to my D8 but it is better to get the bike with the components that you want than to upgrade.