Moving primary key constraint from one index to another
The following script illustrates an efficient way to convert the existing nonclustered primary key to clustered, and to rename it:

-- How the table looks now
CREATE TABLE dbo.Example
(
    pk integer NOT NULL,
    some_data integer NOT NULL,

    CONSTRAINT PK_UnusualName
        PRIMARY KEY NONCLUSTERED (pk)
);

-- Some data
INSERT dbo.Example (pk, some_data)
VALUES (1, 100), (2, 200), (3, 300);

-- Change the nonclustered PK to clustered
CREATE UNIQUE CLUSTERED INDEX PK_UnusualName
ON dbo.Example (pk)
WITH (DROP_EXISTING = ON);

-- Rename
EXECUTE sys.sp_rename 
    @objname = N'dbo.Example.PK_UnusualName',
    @newname = N'PK__dbo_Example_pk',
    @objtype = 'INDEX';

-- Tidy up
DROP TABLE dbo.Example;