Is the provider already enrolled? If so then you need the 855R for reassignment of benefits if your provider is eligible. If the provider isn't you'll need to do both the 855I and 855R. In PECOS if the provider is enrolled just login and I believe it's under "My associates", then you would view or manage enrollments and somewhere in there you can add/change or remove.
Keep in mind if you are a billing company many legal experts and the OIG caution providers on reassignment of Medicare payments due to increased liability to the provider.