Try to the following PowerShell script. It has never failed on my environment.
# Define the SQL command to run
$sqlCommand = new-object System.Data.SqlClient.SqlCommand
$sqlCommand.CommandTimeout = 120
$sqlCommand.Connection = $sqlConnection
Write-Output "Issuing command to scale down database"
# Execute the SQL command
$sqlCommand.CommandText= "ALTER DATABASE [YourDatabase] MODIFY (EDITION = 'Premium', MAXSIZE = 1024 GB, SERVICE_OBJECTIVE = 'P6');"
$result = $sqlCommand.ExecuteNonQuery()