Why? As I just mentioned in another thread. They just download the Beta profile, install the beta then restore from a backup. Apple have worked on the new iPad a lot longer than 11.3. So the two will be ok to work together and apple just need to add the device to list of supported devices.
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He's saying that if the iPad was on 11.2.6 then someone who has an 11.3 beta backup couldn't restore. Which is the correct way round. But they would just install the beta first as I mentioned above.