DataGrid AutoCompleteBox

I have a DataGrid which imports the line items (Partnumber, etc) into a DataGrid - it loads fine - looks perfect.

Now the problem Is I need it to be editable - and I want it to have an AutoCompleteBox in the PartNumber column.

I have 2 DomainDataSource's - One which contains the data for the order in the DataGrid ("POContent") - and one which contains all of the PartNumbers which loads into the AutoCompleteBox ("GetPOPartNumbers").

Every time I hit "SubmitChanges" - no changes are detected.  What I need is the new PartNumber the client chooses in the AutoCompleteBox (aka: PartNumber1) to be mapped to the "PartNumber" in the DataGrid (aka: PartNumber) - and then to have the changes submitted.

From what I can tell because I'm selecting a PartNumber from the AutoCompleteBox that is connected to PartNumber1 - so when SubmitChanges is used it's not using the new part number and commiting to "POContent" - it's only connected to "GetPOPartNumbers" which has no changes.

Any help - much appreciated - below is my current code.

<sdk:DataGridTemplateColumn Header="Part Number" Width="200">
                                <sdk:DataGridTemplateColumn.CellTemplate>
                                    <DataTemplate>
                                        <TextBox Text="{Binding PartNumber, Mode=TwoWay}"/>
                                    </DataTemplate>
                                </sdk:DataGridTemplateColumn.CellTemplate>
                                <sdk:DataGridTemplateColumn.CellEditingTemplate>
                                    <DataTemplate>
                                        <sdk:AutoCompleteBox ItemsSource="{Binding Data, Source={StaticResource GetPOPartNumbers}}" ValueMemberBinding="{Binding PartNumber1, Mode=TwoWay}" IsTextCompletionEnabled="True" Width="200" Text="{Binding Path=PartNumber1}">
                                            <sdk:AutoCompleteBox.ItemTemplate>
                                                <DataTemplate>
                                                    <TextBlock Text="{Binding Path=PartNumber1}" />
                                                </DataTemplate>
                                            </sdk:AutoCompleteBox.ItemTemplate>
                                        </sdk:AutoCompleteBox>
                                    </DataTemplate>
                                </sdk:DataGridTemplateColumn.CellEditingTemplate>
                            </sdk:DataGridTemplateColumn>