Hi @sql user ,
Thanks for using Microsoft Q&A !!
Yes, you can use Azure Data Migration Service to migrate from Azure Database for PostGreSQL server instance to either a single server or Flexible server instance.
However, you need to make sure that PostgreSQL version is 9.x or later. Additionally you need to create instance based on premium pricing tier if you planning for an online migration.
Please refer to Migrate/Upgrade Azure DB for PostgreSQL - Single Server to Azure DB for PostgreSQL - Single Server online for migration steps.
Please let me know if you have any other questions.
Thanks
Saurabh