I don't believe it's possible, as the case for each model is slightly different. A quick eBay search will reveal that there are several different part/model numbers on cases from different "generations". I do not own a Mac Pro, so I cannot verify this, but I have worked on several PowerMac G5's (and many other Macs) and they use different cases for different configurations. Ex: A Mid-2004 (Air Cooled) G5 tower has different motherboard mounting posts than a late 2005 (Liquid Cooled) model, making installing a different motherboard impossible. I would assume that the same applies to Mac Pros, but anyone who has more experience with Mac Pros should feel free to correct me.