Configure the Tab Key to Apply Multi-Level Paragraph Numbering in Word 2003
Prior to Word 2003, you could use the [Tab] key within a numbered list to “demote” items to the next level of numbering and use the [Backspace] or [Shift]+[Tab] to “promote” a paragraph to the next higher level. This feature was turned off in Word 2003 most likely to encourage the use of styles to apply your numbering format, of which I am a proponent.
However, if you want the [Tab] and [Backspace] demote/promote functionality feature turn back on, go to Tools > Autocorrect Options and make sure “Set left- and first-indent with tab and backspace” is checked.
For more information about using styles when numbering paragraphs, see Microsystem’s The Seven Laws of Word’s Outline Numbering: What to do — and not do — when using Word’s outline numbering linked to styles.