The graphics seem tweaked for no good reason, and a lot of the designs are getting reworked. Why? Who knows, it's probably for the same reason 343 did it: because they can.
I haven't touched the game, I've only seen as much as most people on the forums. My biggest worry is a redo of year one, an absolute mess, but it looks like all the quality of life updates we've been asking for are going into the sequel (which is ass imho).
[spoiler]tl;dr I want more gameplay man I gotta see what I'm buying first [/spoiler]
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I'm sure there will be an open beta. If that's not enough, just watch for however long you need to after the release.