I think I looked into this a bit back and changing the number now could void your contract. I think only Sprint allows you to change mid-contract.
Not sure what happens if you are out of contract. I assume it'd be fine, but you'd have to tell everyone you new phone number and you wouldn't have any cushion with your old number.
I've found that using both numbers is fine on my phone. As long as people remember to dial the right number, the best part of Google Voice is the voicemail to email/Google voice app. When I had poor cell phone reception at my old apartment the gmail calling was a God send as well.