Hi @Roger Roger Thank you for posting your question on Microsoft Q&A and for using Azure services.
My understanding is that you want to know the difference between Production (Small/Medium-size) & Production (Large-size) and also if it will be possible to change Production (Small/Medium-Size) to Production (Large-size).
The Production (Small/Medium-size) is best suited for production, pre-production or testing **small to medium size databases with low-medium concurrency requirements. **
The Production (Large/Medium size) is best suited for production, pre-production or testing **large databases with high concurrency and availability requirements. **
To Covert to large-size production you can perform scaling operations for compute and storage. You will be able to change your compute tiers between burstable, general purpose, and memory optimized SKUs, including choosing the number of vCores that is suitable to run your application. see Scale Operations in Flexible Server.
The General Purpose and Memory Optimized are better suited for production workloads requiring high concurrency, scale, and predictable performance. You can build your first application on a small database for a few dollars a month, and then seamlessly adjust the scale to meet the needs of your solution. See Overview - Azure Database for PostgreSQL - Flexible Server
See also Azure Database for PostgreSQL pricing for more details
Please let me know if you need additional clarification