Basically, user has updated a control that is bound to a table or query field, then has done something to cause the update event to run (if this is a combo, likely user has made a selection from the combo list). So the value that it once had is different from the value it now has, but the new value has not been committed/changed. You cannot edit the value before this happens. You could do it in the After Update event though - unless an update results in a list item value that violates what the table field allows.
EDIT - Actually I don't think you can change the case of a list item if the list comes from a table or query. You'd have to have the different case options be list items. You might be able to edit a list item if the combo is based on a value list and not a table/query field. However this will permanently alter the value until it is altered again.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.