I think I'd try the CustomValidator. With the CustomValidator, you can define the validation code in the "_ServerValidate" event handler.
In your validation code, you can call the database to get the current value and then compare it to the submitted value (which would be available via the event handler's ServerValidateEventArgs paramenter if you specify a ControlToValidate).
In order to look up the current value in the database, you'll need to know which record was clicked. For that, you could either use the row button's CommandArgument property or include a hidden field in your row to store some type of ID. Then you could capture that ID and use it to look up the desired record in the database.
Updated with Code
Protected Sub cusCustom_ServerValidate(source As Object, args As System.Web.UI.WebControls.ServerValidateEventArgs)
Dim cv As CustomValidator = CType(source, CustomValidator)
Dim HiddenField_ID As HiddenField = CType(cv.Parent.FindControl("HiddenField_ID"), HiddenField)
*** Use ID to look up value in database ***
End Sub