Not exactly. Apple could update the GPUs on the Tube every 6 months if they wanted. The problem is on the user side: the GPU boards are not servicable by the average user. Even if Apple boxed up new GPU boards into pretty white boxes and sold them off the shelf at Apple Stores, there is no chance in hell they would let users install them. It is a hundred times more likely that if you check an Apple Store tomorrow you will find i7 xMacs for sale stuffed full of GTX 1080 video cards.
First, replacement of the boards requires a complete teardown. The whole freakin' Tube must come apart.
Second, it's not as if each GPU board has it's own little heatsink to keep it cool, all of the processors in the Tube share one big honkin' heatsink. So to replace the GPUs, there is not only the teardown but also cleanup of the old thermal paste, then reapplication of new paste and proper assembly of the GPU against the heatsink.
Ask users to do this and there will be ESD damage, farked cables and connectors, buggered thermal grease application, and completely FUBARed CPU sockets. "But they don't need to remove the CPU from the socket" you say? Ask them to replace the GPUs and I guarantee you that some portion of user attempts will result in removal of everything but the GPUs.
I describe this in detail because at the time of the Tube's launch Apple fanbois descended on this forum like a horde of locusts to plague us with talking points, the most obnoxious being that "the GPUs really are upgradable!" They argued that Apple would provide upgrade GPUs after a year or so. I thought they might be bots until I realized the monumental challenge of programming such breathtaking stupidity into a bot.
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Just screwed, I've had a few Tubes open and Apple doesn't glue that model together. Not yet, at least.