Returning old router and using new router before moving

I have received a new Xfinity router for use with a new account at the house where I am moving to soon. But I am still using an old Xfinity router at my current address for the moment, until my current service ends. Can I use the new Xfinity router now, or must I wait until actually moving? How long do I have before having to return the old router -- from when the new service starts, or from when the new router arrives? Thank you.