My advice would be do it now, while she's still little. That way, she can assimilate more easily into the next group. If you think back to elementary, remember how kids usually played within their own age group? You don't want her to feel ostricized, and I think that is more likely to happen if you wait and do it later.
It's too bad they didn't see her abilities in kindergarten and just bumped her up to first grade then... would have meant almost no transition.
We had a girl in our class who was moved up before she transfered to our school, and you never knew. She fit in fine. I only know now because it was mentioned in high school.
I've had classmates who also stayed in our elementary class, but attended a higher grade for a certain subjects too, so maybe that's an option if you don't want her to leave her group all together.
I'm for it, if appropriate for your daughter and her situation. The last thing you want is a child who gets bored with school and stops paying attention simply because they aren't challenged enough. She may always be ahead of the class she's in, and the teacher may not always be able to dedicate the attention she needs to nurturing her abilities when they have a whole class to cater too...
Just my relatively uninformed opinion!
good luck!