I assume you are asking about MongoDB Atlas and are currently using the free tier.
If you are using a shared tier cluster (M0, M2, M5) the maximum storage is a hard limit that cannot be exceeded: your application will receive a write exception if a write would exceed your cluster’s current quota.
There is also a new Atlas Serverless offering currently in preview, which has pricing based on resource usage (reads, writes, and storage) rather than cluster tiers based on instance sizing (RAM & CPU).
Storage is an ongoing cost rather than a one-off upgrade. If your storage requirement is TBs of data, you would either have a dedicated cluster (with options like auto-scaling) or be using Serverless (usage-based pricing).
Ok let me understand you, first I upgrade to a cluster of m10, it is 0.08 dollars per hour with the max value of 128GB, per month it would be 56.7 dollars.
So suppose I update the cluster, now if my db is 2gb and suddenly I don’t pay for the m10 cluster, they change me to m2 (current), then mongo atlas would send an answer error: your database has exceeded 512 mb.