I just received an email from Sprint that if I turn in my current iPhone XR, I can upgrade to the new iPhone 12 mini for free (with 24 months of bill credits). No new line required. Here is the email:
[INDENT][I]Upgrade to iPhone 12 mini On Us with eligible device trade in Enjoy the superfast 5G speeds1, enhanced camera capabilities, and next-level features on an epic new iPhone.
Pick yours now
Offer ends soon!

1. Data plan required. 5G and LTE are available in select markets and through select carriers. Speeds vary based on site conditions and carrier. For details on 5G and LTE support, contact your carrier and see apple.com/iphone/cellular.