How you will do this will depend on how you have this information stored in the db. If you have each answer that the user gives as a unique record in the db then it might be a little simpler to implement.
From what I can tell you will need to have some custom parts for this. The first part would be creating a recordset. You can then populate the form elements with the values from this recordset to show the answers that have already been stored.
Next you would have the user make the updates that they need and submit the form. You would need to loop over the posted values and compare them to the values in your original recordset. If the value does not match the original value then you will update the record. When you update the value with the posted value you can also update the time field with the current time.
This is a custom thing but I think this is the best approach. This way you can make use of a single update server behavior and update only the records that have changed. If you tried to do this with a multiple update server behavior all of the records would be updated regardless of weather or not the value changed. You will need to manually check the value against the existing value to ensure that it has changed.
