What's wrong with my numbering in microsoft word 2007?

When I had a numbered list in microsoft word, I could hit the tab key to change the numbers to letters, then enter to change them back, but I am unable to change the numbers to letters with the tab key anymore. what's wrong?What's wrong with my numbering in microsoft word 2007?
Presumably you are using the number icon, not typing numbers, but even so, I've never known that, because the tab key moves you along to the next tab setting , or more than likely indent the whole line, but if you are in a table environment and have numbering in first column, tabbing takes you to the next column/he next cell. The way to change format of numbering or bullets is right button of mouse down to, in your case, numbers and choose the format from the window or click on the customise button. With 2007 it is the same, right button, choose another numbering format from the gallery of options or Define new number format.