I have a M0 Atlas cluster which runs on 4.4 drivers at the moment. I received an email about an automatic upgrade later this month, but I would like to perform the upgrade to the 5.0 drivers manually, so that I’m able to fix any problems that may occur.
Is it possible to do? I tried to click “upgrade” in the Atlas panel, but it only allows to upgrade to M2+, claiming there is only one M0 cluster available.
I don’t believe there is a way to choose the version that the M0 tier uses. The version and upgrade are controlled by MongoDB since it’s the free tier.
MongoDB Version Upgrade You can’t upgrade the MongoDB version that M0/M2/M5 clusters run.
Atlas upgrades M0/M2/M5 clusters to the newest MongoDB version after several patch versions become available for that that version. To learn more about how MongoDB versions its software, see MongoDB Versioning.
As @tapiocaPENGUIN noted, M0 and other Atlas shared clusters (M2, M5) currently receive automatic server version upgrades. Per the email communication you received, the major version upgrade to MongoDB 5.0 will happen this month across the global fleet of shared tier servers.
MongoDB driver versions are compatible with a range of server versions, so if you upgrade to the latest driver for full MongoDB 5.0 feature support, this driver should also provide support for previous non-EOL server releases (at least as far back as MongoDB 4.0 at the moment).
Upgrading to the latest driver version is highly recommended for full compatibility, but is not a strict requirement. If your application is using a slightly older driver (for example, one including full 4.4 support) I expect you are unlikely to encounter any issues aside from lack of support for new server features. If you are using a much older driver (for example, one targeting 4.0 support) you are exposed to a much broader range of server changes. MongoDB 5.0 also introduced a new Stable API (originally called the Versioned API) that provides a guarantee of future server compatibility so you can Upgrade Fearlessly if you adopt it.
The MongoDB server release notes include Compatibility Changes in MongoDB 5.0 to be aware of. Many of those are specific to server configuration and would not apply to the Atlas shared tier.