I recently updated the rancher catalog WAOS to upgrade MongoDB 4.2 to 4.4. I was surprised to find that rancher 2.4.3 did not handle helm type catalog upgrades properly. So I upgraded rancher 2.4.3 to 2.4.5. Here are the steps performed.

# upgrade
helm repo update
helm fetch rancher-latest/rancher
helm get values rancher -n cattle-system
helm upgrade rancher rancher-latest/rancher \
  --namespace cattle-system \
  --set hostname=MYHOSTNAME

This was added to the rancher snippets. Be careful; the latest version is not a stable production.

After a catalog refresh from rancher interface, you should be offered a Mongo update :

Back up your data then click on it, select 4.4 and upgrade.

It's that simple. Mongo instances will be upgraded one by one. The key is to regularly update updates to keep up with new developments and not have to overcome upgrade walls later.

Welcome to the long quiet river of rancher responsiveness :)