now that i had a sip of coffee, it's even easier:

create the new table on the fly:

--script out the constraints and indexes from the original table

--create the new table instantly with the data and the identity column.

SELECT

identity(bigint,1,1) AS NewColumnName,

ColumnList

INTO OriginalName_TMP

FROM OriginalName

--rename the original

EXECUTE sp_rename 'dbo.OriginalName','OriginalName_SAV'

--rename the new table to teh right name

EXECUTE sp_rename 'dbo.OriginalName_TMP','OriginalName'

--modify the constraint script to have unique names for the new table and run them

ALTER TABLE ADD CONSTRAINT UQ_...

CREATE INDEX....