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I have bought a new Apple Watch Series 3. However, I want to go ahead and replace it with a aftermarket housing. Problem comes in, that these housings were introduced when Apple had just recently introduced Apple Watch. My question is, is there a huge difference in those housings? If not, can I replace the housing made for Apple Watch, 1, or 2 into the Series 3 Apple Watch?

Yes there are differences, not only just size differences, but where screw holes are etc. You wouldn’t be able to use an older model housing with series 3.