Change Datatype of Datacolumn

converting datatype of datacolumn in a datatable at runtime can be done in two methods
1) while filling in adapter
da.SelectCommand = cmd
da.FillSchema(dt, SchemaType.Source)
dt.Columns(6).DataType = GetType(String)
dt.Columns(7).DataType = GetType(String)
dt.Columns(8).DataType = GetType(String)

da.Fill(dt)
2) if your datalayer returns only datatable then here is the second method
Dim tempdt As DataTable = dt.Clone
tempdt.Columns(6).DataType = GetType(String)
tempdt.Columns(7).DataType = GetType(String)
tempdt.Columns(8).DataType = GetType(String)
For Each r As DataRow In dt.Rows
tempdt.ImportRow(r)
Next