I have no reason to believe, nor have I heard, that they would be thinking about ending/changing Extra Magic Hour/Magic Morning for hotel guests and those with appropriate entrance media. I suppose they could change the days around, but I wouldn't expect anything beyond that. That being said... There's only 2 areas that are open for EMH/MM at
Disneyland, and those are Fantasy Land and Tomorrow Land. Without them opening other areas, it would be pretty difficult to allow people into Galaxy's Edge. I suppose you could have people take the Big Thunder Mountain Trail from Fantasy Land to get there.
In other words, I wouldn't make an on-site reservation in an attempt to get into Galaxy's Edge a little earlier, at least until time gets closer and we have an idea of what they are going to be doing.
But if you'd be booking on-site anyway, then it can't really hurt. Just as long as you aren't doing the reservation specifically for early access. But, you could cancel anyway, I suppose, if it becomes obvious that they aren't opening Galaxy's Edge for EMH/MM.