Regarding role=tooltip and event firing for dynamic tooltips

Hi,
I was doing a bit of experimentation to see what would happen if role=tooltip were applied to a triggering element such as an edit field, and I dynamically updated aria-describedby to point to a different tooltip with a different ID while typing.

The scenario being a dynamic help tooltip where the user receives helpful feedback while in the process of entering data to shape their response.
E.G http://whatsock.com/tsg/Coding%20Arena/Inline%20Form%20Field%20Validation%20and%20Dynamic%20Help%20Tooltips/Dynamic%20Help%20Tooltip/demo.htm

Using role=tooltip + aria-describedby doesn't work for this scenario however, since the newly added aria-describedby value isn't announced when the tooltip changes to another.

So basically, is this due to an event not firing in the browser, or because the AT isn't recognizing the change?

Btw, the referenced url above doesn't use role=tooltip for this reason, but I can make a demo that does if that helps for testing.

Thanks,
Bryan