Data Warehouse: Remove legacy data after conversion into new system
I would suggest dropping the database once it is no longer needed.

DROP DATABASE LegacyDatabase_0001;
DROP DATABASE LegacyDatabase_0123;


The database space is freed for the use of other expanding databases and/or for creating further new databases.

If you are concerned about data loss you could archive the LegacyDatabases to some backup location, a file share, tape backups, whatever your choice may be.