SQL script help: "Moving a database"

I'm trying to develop a script to change the drive letter where my SQL server looks for its databases.  I've cobbled this together, but I can't figure out my way around this error:

DECLARE @DBName varchar(50) = '';
DECLARE @NewMDFName varchar(50) = '';
DECLARE @NewLDFName varchar(50) = '';
DECLARE @OldMDFName varchar(50) = '';
DECLARE @OldLDFName varchar(50) = '';
DECLARE @LogicalName varchar(50) = '';

SET @DBName = 'TestLun1a';

SET @NewMDFName = @DBName + '.mdf';
SET @NewLDFName = @DBName + '.ldf';

USE Master;

SET @OldMDFName = (SELECT name FROM sys.master_files WHERE database_id = DB_ID(@DBName) AND type_desc = 'ROWS');
SET @OldLDFName = (SELECT name FROM sys.master_files WHERE database_id = DB_ID(@DBName) AND type_desc = 'LOG');

ALTER DATABASE [@DBName] SET OFFLINE WITH ROLLBACK IMMEDIATE;

ALTER DATABASE [@DBName] MODIFY FILE ( NAME = [@OldMDFName], FILENAME = 'F:\' + @NewMDFName] );
ALTER DATABASE [@DBName] MODIFY FILE ( NAME = [@OldLDFName], FILENAME =  'F:\' + @NewLDFName] );

ALTER DATABASE [@DBName] SET ONLINE;

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The script is supposed to unmount the database, update SQL Server on its "new" location, then mount the database again.  Any help would be appreciated.