She's a pretty little girl, congrats.
Breeders can NEVER accurately predict weight...it's simply a guess. Dogs almost never do what you think they're going to...they do what they WANT to. Most breeders have something to that effect in their contract. You just never know. It's actually VERY generous that the breeder would allow you to switch dogs at this point.
Of course, 6 lbs at 6 months doesn't mean she'll be beyond the standard. As long as she's 7 lbs and under she's fine. Usually small breeds are 90% or more of their size by 6 months...but please DON'T QUOTE ME ON THAT!
