From everything I've heard, theoretically it shouldn't make a difference when you upgrade. But, as a practical matter, it seems to work so much smoother at the guest services inside the park after you've entered the park at least once.
Don't know if it's important, but you can use you're 3 days during blackout days for the power pass and still upgrade to the power pass. Just make sure you don't need to reenter the park on a blackout date after upgrading. I entered the park January 2nd, park hopped, then upgraded to a power pass inside the park. I could stay in the park. But, I wouldn't have been able to park hop again or leave and reenter.