Hi,
Looks like it can be done while the databases are online:
Partitioned Tables and Indexes
https://learn.microsoft.com/en-us/sql/relational-databases/partitions/partitioned-tables-and-indexes?view=sql-server-ver15
Create Partitioned Tables and Indexes
https://learn.microsoft.com/en-us/sql/relational-databases/partitions/create-partitioned-tables-and-indexes?view=sql-server-ver15
Best regards,
Leon