It depends on the beer. It does change the beer, maybe not in the flavor but in the mouthfeel and body so it's not like the beer is exactly the same but with a bit more ABV. There is a difference that is usually noticeable. That can be a good thing- (a Belgian tripel, an IIPA)- or it could be a bad thing (stout).
As far as flavor, you can use quite a bit without impacting the flavor but I've never gone over about 10% in a tripel.
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