I haven't tried, but are you sure you can convert a uniqueidentifier to a bigint?
Yes, but not directly.
SELECT CAST(CAST(NEWID() AS BINARY(16)) AS BIGINT)
--Jeff Moden
RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row. First step towards the paradigm shift of writing Set Based code: ________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.
Change is inevitable... Change for the better is not.