Forms Data Controls :: Gridview Copying Data From One Field To Another When Updating

I have a gridview based on a sqldatasource that has some colums from which two colums showing the data of two fields: field1 and field2. field1 is an ID which should not be updated and therefore it is readonly=true. The sql-query from the sqldatasource is:

SelectCommand="SELECT field1, field2 from anyView"

What I want is, that when a row is updated, then the value from field1 should be copied to field2 The update-query is:

UpdateCommand="UPDATE myTable SET field2=@field2 WHERE field1=@field1"

My idea was, that the @field2 paramter gets the value from the query string out of field1:

<UpdateParameters>
<asp:QueryStringParameter Name="field2" QueryStringField="field1" Type="Int32" />
</UpdateParameters>

But this does not work. Is there an easy way to copy the data from field1 to field2 when updating the row? In addition, is this possible, even if field2 ist not shown in the gridview (i.e. field2 is selected in the query but is not a field in the gridview but should be updated with value of field1).