Hmm, I'm not sure if you truly meant to "sort" the combo or change the bound column. If you sort the combo using a column, which is not the leftmost visible column, then I am not sure how it will help the user look for the item they are sorting
by because entering data to the combo refers to the first visible column.
In some cases like this, I have used separate comboboxes with each one displaying different columns as the first column, so the user can use any of the combos but still store the same data into the table/form. As soon as the user selects an item from one
of the combos, the other combos follow suit.
Just my 2 cents...