How do I rename my constraints

After some more digging, I found that it actually has to be in this form:

EXEC sp_rename N'schema.MyIOldConstraint', N'MyNewConstraint', N'OBJECT'

Source

I am not a big fan of cursors and this can be written much more simply.

DECLARE @SQLCmd varchar(MAX) = ''
SELECT 
    @SQLCmd += 'EXEC sp_rename ''' + dc.name + ''', ''DF' + 
                OBJECT_NAME( dc.parent_object_id ) + c.name + ''', ''OBJECT'';'
FROM 
    sys.default_constraints dc
    JOIN sys.columns c 
        ON c.object_id = dc.parent_object_id 
        AND c.column_id = dc.parent_column_id
WHERE 
    dc.name != 'DF' + object_name( dc.parent_object_id ) + c.name 
    AND OBJECT_NAME( dc.parent_object_id ) != 'dtproperties'
EXEC( @SqlCmd ) 

Try:

exec sp_rename 'FK_tblOldAndBusted_tblTastyData', 'FK_tblNewAndShiny_tblTastyData', 'object'

Also, there is a bug regarding renaming such things when you deal with non-default schema.

Cannot rename a default constraint for a table in a schema which is not dbo by rsocol @Microsoft Connect