How to upgrade a version to a cluster in Mongo with the creation of a replica without losing data while moving between the clusters
Hello @Ruth_Goldstein ,
Welcome to The MongoDB Community Forums! 
You should not lose data while you upgrade to a new version of MongoDB. In case you face any issues, I would advise you to bring this up with the Atlas support team or connect with support via the in app chat support available in the Atlas UI. They may be able to check if anything on the Atlas side could have possibly caused this issue. In saying so, if a chat support is raised, please provide them with the following:
- Cluster link / name which experienced the issue
- Time & date including timezone for when it occurred
- Any error message output
- Any additional details that you think will help them point to the issue
Below is the recommended procedure on how to Upgrade Major MongoDB Version for a Cluster
In case you want to modify your current cluster, below documentation can help you in that case
Regards,
Tarun