Is it possible to loop through a filtered data table and update the items as needed?

In my ‘For i As..Next’ loop, I want to check the value of a specific column in the next row and if they’re not equal to the column in the current row, perform some calculations. I’ve tried using this approach in a ‘For Each’ loop, but I don’t believe it’s possible? Also, I’m trying to only loop through records I care about (specifically the ones that result from my SelectStatement parameter) like I’m doing with my first ‘For Each’ loop.

Public Shared Function MyFunction(ByRef TimeDataTable As DataTable, ByRef SelectStatement As String) As Boolean
    Try
        Dim Units As Decimal                    ' units (hours) billed for this day
        Dim Hours As Decimal = 0.00             ' total hours minus any PTO
        Dim ProratePercent As Decimal = 0.00    ' calculated percentage; 40+ hours
        Dim NewUnits As Decimal = 0.00          ' new hours after calculating with ProratePercent

        ' prorate employee's hours for the given week
        For Each TimeRow As DataRow In TimeDataTable.Select(SelectStatement)
            Hours = (TimeRow.Item("TotalHours") - TimeRow.Item("PTOHours"))
            Units = (TimeRow.Item("Units"))

            ProratePercent = CDec(Units / Hours)
            NewUnits = (ProratePercent * (40 - TimeRow.Item("PTOHours")))
            TimeRow.Item("Units") = Math.Round(CDec(NewUnits), 2)

            'remove this before commiting to production; just to mark that this was manipulated in the import file for data comparison
            TimeRow.Item("Time Off Name") = TimeRow.Item("Time Off Name") & " (Prorated)"

            Hours = 0.00
            ProratePercent = 0.00
            NewUnits = 0.00
        Next

        ' check previous usernames; if they don't match, check if last value adds total prorated hours to 40
        For i As Integer = 0 To TimeDataTable.Rows.Count - 1
            Dim TimeRow As DataRow = TimeDataTable.Rows(i)
            Dim Username As String = TimeRow("Username").ToString()

            If i <> TimeDataTable.Rows.Count - 1 Then
                Dim nextRow As DataRow = TimeDataTable(i + 1)
                Dim NextUsername As String = nextRow("Username").ToString()
                If Username <> NextUsername And (TimeRow("TotalHours") > 40) Then
                    ' adjust the final record to equal 40 hours
                End If
            End If
        Next

    Catch ex As Exception
        _logger.Error("ProrateUnits function error: " & ex.Message.ToString())
        Return False
    End Try

    Return True
End Function

I don’t like how I’m resorting to looping through every single record in the data table in this last ‘For Next’ loop.

Visual Studio skips over my logic whenever I try to use this:

            For i As Integer = 0 To TimeDataTable.Select(SelectStatement).Count - 1
                Dim TimeRow As DataRow = TimeDataTable.Rows(i)
                Dim Username As String = TimeRow("Username").ToString()

                If i <> TimeDataTable.Select(SelectStatement).Count - 1 Then
                    Dim nextRow As DataRow = TimeDataTable(i + 1)
                    Dim NextUsername As String = nextRow("Username").ToString()
                    If Username <> NextUsername And (TimeRow("TotalHours") > 40) Then
                        ' adjust the final record to equal 40 hours
                    End If
                End If
            Next