declare @poId int
set @poId = 130153;

with ChequeNumChanges(ModifiedDate, Val1, Val2, rN) as(
    select poDate, poChequeNumber, null, 0 from dbo.PurchaseOrders where poID = @poId union all
    select ModifiedDate, OldValue, NewValue, ROW_NUMBER() over(order by id) as rN
    from dbo.POAudit where poId = @poId and FieldName = 'poChequeNumber'
)
select * from(
    select case when c1.rN = 0 and c2.ModifiedDate is null then c1.Val1
                when c1.rN = 0 then c2.Val1
                else c1.Val2 end as num,
    c1.ModifiedDate as DateIn, c2.ModifiedDate as DateOut
    from ChequeNumChanges as c1
    left join ChequeNumChanges as c2 on c1.rN = c2.rN - 1
) as t
inner join dbo.POAudit as a on t.num = a.ForeignKey and a.ForeignKeyTable = 'c'
--where a.ModifiedDate >= t.DateIn and (a.ModifiedDate < t.DateOut or t.DateOut is null)