What age? 10u? If your new bat has a warranty, I'd say use it for practice and games. Maybe use the old bat on days where the temp drops below 40 degrees. I had a 30/19 Demarini CF8 survive 2 DDs in 10u, then sold it to another dad for his 10u, and I believe it is still going strong.
My daughter has a cage bat. A DeMarini rep near us was selling their "trial" bats for $65. Picked up a brand new Phoenix and one that has seen better days. We were just going to get one but we couldn't pass up the new bat.
She uses the older one in bad weather and the cages.
Thanks for the advice. She's been using a 2013 Vendetta for BP which she used in games until her current Vendetta, that has gone through a few other girls.