the on change or after update actions are just that actions. It is something that happens to the box. If the text box is a formula and there is no phyisical click or manipulation I dont think it will register as a chane/update.
maybe you can put the code in the on get focus option. Then code the text box to get focus and then code what ever else you need to get focus so taht the end user neaver notices. Maybe this will help, though someone might have a better idea