Index on a highly updated field causing excessive locks in SQL server
Instead of using the DateChanged column in the where clause perhaps you could add an IDENTITY column, and use it in the .Net client to keep track of processed rows?  The .Net client could periodically save the last processed identity value in case it needs to restart.

This would allow you to get rid of the index on DateChanged completely, and queries getting the new rows would be fast since they would be performing a range scan on values greater than the last processed identity value.  Something like:

SELECT 
  [TrackSessionID],
  [DataPartID], 
  [UserMasterID], 
  [RecordID], 
  [DateChanged] 
FROM dbo.TrackSessions 
WHERE [TrackSessionID] &gt; @LastTrackSessionID 
ORDER BY DateChanged DESC;


A potential downside to this approach might be the IDENTITY column becoming a source of contention for inserts into the table.  That could potentially be mitigated by using several partitions with a partition function that uses a hash function to decide which partition each insert goes into.

Something like this comes to mind:

CREATE PARTITION FUNCTION pf_TrackSessions (INT)
AS RANGE RIGHT FOR VALUES (1, 2, 3);

CREATE PARTITION SCHEME ps_TrackSessions
AS PARTITION pf_TrackSessions
ALL TO ([SessionFileGroup1]
    , [SessionFileGroup2]
    , [SessionFileGroup3]
    , [SessionFileGroup4]);

CREATE TABLE dbo.TrackSessions
(
    TrackSessionsID INT NOT NULL IDENTITY(1,1)
    , TrackSessionsIDModulus AS (TrackSessionsID % 4) PERSISTED
    , DataPartID INT NOT NULL
    , UserMasterID INT NOT NULL
    , RecordID INT NOT NULL
    , DateChanged DATETIME2(7)
        CONSTRAINT DF_TrackSessions_DateChanged
        DEFAULT (GETDATE())
    , CONSTRAINT PK_TrackSessions 
        PRIMARY KEY CLUSTERED
        (TrackSessionsIDModulus, TrackSessionsID)
) ON ps_TrackSessions (TrackSessionsIDModulus);
GO


The above DDL requires 4 filegroups, one for each partition.  Rows would be evenly distributed among the 4 filegroups based on the modulus of the identity value.  This reduces contention against the allocation bitmap pages by splitting the load across 4 sets of each structure.