Fun with NEWSEQUENTIALID()

We’re thinking about converting the constrain on an existing column in a table with billions of rows of data.  The column is a uniqueidentifier serving as a nonclustered primary key; we’re planning on replacing the NEWID() constraint to a NEWSEQUENTIALID().  What is challenging me is my concern over primary key collisions; will the new default values eventually run into an existing primary key value?

I used the following script to test, and I was a little surprised at the results:

CREATE TABLE TestNewSeqID(ID uniqueidentifier CONSTRAINT DF1 DEFAULT newID(),
                            Marker integer Identity(1,1))
GO

DECLARE @x int
SET @x= 1
WHILE @x <=10
BEGIN
    INSERT INTO TestNewSeqID DEFAULT VALUES
    SET @x = @x +1
END


SELECT *
FROM TestNewSeqID
ORDER BY ID DESC


ALTER TABLE TestNewSeqID
DROP CONSTRAINT DF1;

ALTER TABLE TestNewSeqID
ADD CONSTRAINT DF1 DEFAULT newsequentialid() for ID;

SET @x= 1
WHILE @x <=10
BEGIN
    INSERT INTO TestNewSeqID DEFAULT VALUES
    SET @x = @x +1
END

SELECT *
FROM TestNewSeqID
ORDER BY ID DESC

My results looked something like the following:

ID    Marker
F664E06A-51BF-4FC1-960E-C9E5E854ADAD    9
0746DBAC-8B14-463C-8C4E-9CC789794963    7
24217231-1865-4E2A-B8EF-944DEC9AC6A2    1
323C15B4-59DA-4B73-94E5-54A1A0860F3D    8
B0C1E7EE-3701-4916-A3E2-465FE8462965    2
BFE8C2AB-4788-4596-867E-221F75BAB338    10
C0BC2263-F5E6-41F1-83F6-14BD9D50A6DD    3
9FA2A683-FF02-458E-8703-10D1DCDEFA97    4
75445937-6B46-4B88-9525-0DD809A7BE4B    6
E93284BE-E93B-4009-B206-04CB2B107293    5
4390AD7A-CD7D-DE11-98B8-00196665566A    20
4290AD7A-CD7D-DE11-98B8-00196665566A    19
4190AD7A-CD7D-DE11-98B8-00196665566A    18
4090AD7A-CD7D-DE11-98B8-00196665566A    17
3F90AD7A-CD7D-DE11-98B8-00196665566A    16
3E90AD7A-CD7D-DE11-98B8-00196665566A    15
3D90AD7A-CD7D-DE11-98B8-00196665566A    14
3C90AD7A-CD7D-DE11-98B8-00196665566A    13
3B90AD7A-CD7D-DE11-98B8-00196665566A    12
3A90AD7A-CD7D-DE11-98B8-00196665566A    11

If I looped through several more times, the seed changed and the values got progressively higher, but they were still sorting lower than the original data set.  I’d appreciate it if others would run this script and see if they got the same results.