The company I am working for now faced the same issue.
What we did was have both the mongo drivers installed (mgo and go-mongo-driver). All the newer APIs were re-written to the official mongo driver by the import alias method. Then we gradually re-wrote the existing APIs to the official driver.
The below post was of great help during the migration process:
Unfortunately, there is no straightforward way built-in way of doing this (at least none that I am aware of).
Thank you for your help it was usefull.
I asked this question in mongo-driver jira board and in reply they send to me link to package. This package handle bson from mgo: mgocompat package - go.mongodb.org/mongo-driver/bson/mgocompat - Go Packages
I just set this property to connect and problems with unmushal to mgo bson was resolved.
Simple example usage: connOpt := options.Client().SetRegistry(mgocompat.Registry)