DD isn’t replacing NR in Open edX—I’ve been working on adding some DD and OpenTelemetry support in edx-django-utils, but they would coexist as options. Also see OpenTelemetry Integration Coming Soon! for some related work in the OpenTelemetry space. (In general, we should be moving towards making all of these things be pluggable.)
I don’t have much knowledge on NR integration because it was already set up before I arrived, so all my research has been on DD and OTel. But I believe the newrelic Python package sends APM data directly to NR’s servers, rather than requiring it to go through a local agent like with DD.