No, there is no fee to make this change. Depending on the rate plan you are on, it may need to be changed based on the phone that you are swapping to - i.e., if you have a basic feature phone and swap to a smart phone, you will need to change the plan to match.
Is her old number still active with Sprint? If so, we can do a 'flip/flop' between your current phone/phone number and her phone/phone number. But her line needs to still be active in order to do this.
*edit - sorry, I just re-read your post. Yes, we can change the phones out. I'll send you a private message to help you out.
My daughter had a Sprint phone and a Sprint number until she got an iPhone and a different number/different provider.
Her phone is newer than my existing phone (with an existing Sprint number) so I would like to move my number to her old phone and her old number to my current phone so we can still have both numbers and both phones, but I get the newer phone.
How do I go about that? Do I need a THIRD phone to talk to Sprint with while I'm doing that?