There are two ways to do this:

The Easy Way = The 'letters' must be available as autonumbers associated with a heading level. Then you modify the Page Number settings by going to Insert > Page Numbers > Format Page Numbers... and set it to include that heading level.

The Hard Way requires a macro which retrofits each paragraph in the TOC after that field is updated. This is not a great way to do it so you should try the Easy way first.

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia