I think it is a new aircraft based on the CRJ 700 airframe. I don’t think they are retrofitting old 700’s just using the 700 airframe design and revamping the seating and galley configurations
https://www.bombardier.com/en/media/newsList/details.bca-20180206-bombardier-launches-new-innovative-50-seater-aircra.bombardiercom.html