After looking into it for a bit, I don't think so, but I'm not sure. I don't see any major challenges to putting an upgraded motherboard into an older case, as long as the motherboard is made for a case with a simmilar series. I wouldn't really want to count on it, however, and shuttle doesn't look to be going out of its way to make this easy. Comparing the G4 and G5 series cases and motherboard, the one thing that immediately jumped out at me was the difference in front panel connectors. My G5 case has one short ribbon cable to connect everything on the front panel, while the G4 still has individual cables. Getting a G5 motherboard into a G4 case looks easy aside from this issue, but this issue looks painful. I didn't see any replacement motherboards for my G5 case, so I don't think they're sold seperately from their cases (though there is always ebay...). Considering all the constraints in cooling and custom connectors, I don't think they can garuntee that they can fit the next new processor socket and cooling system into a current case.
Honestly, I wouldn't worry about it unless you're constantly tinkering (and if you are, the cramped quarters are going to annoy you anyways). Considering the cost they'd likely charge you for the new motherboard, I'd just suck it up and buy the rest of the case at the same time. (Or my personal favorite, pay a bit more and end up with two working computers. My family likes it when I do that...)
Discussion Zen 5 Speculation (EPYC Turin and Strix Point/Granite Ridge - Ryzen 9000)
Discussion Intel current and future Lakes & Rapids thread
