It will be much easier to re-engine current types as they were designed for modern high bypass engines, unlike the B737 which was built for low bypass JT8D motors. Installing unducted fan engines could prove problematic given the differing aerodynamics, and would probably be best on a clean sheet design drawn up with UDFs specifically in mind.
The USAF is going to re-engine it’s B52s and was originally going to use 4 high bypass turbofans but decided to continue with the original 8 engine configuration to reduce the amount of structural modifications required.
https://www.ainonline.com/aviation-n.../new-life-b-52