>>
>>If the only valid choices are "A" or "I", why would you need to delete the value?
>
>When I want to change from A to I, from user's point of view I have to delete "A" and type "I". However, I can not delete, I can only type "I", which is not 100% intuitive. Anyway, I can live with this.
>

I think it is intuitive, even from a user standpoint. I guess it could be unintuitive if the flashing cursor is after the letter, so they might think they need to back up. Perhaps you could use the selStart and selLength properties of the textBox to keep the text selected, so it is more obvious. Or, use the same properties to keep the cursor before the letter.