Like many problems with Word 2010, this one can be resolved by the right-click/context menu method. If you right-click on one of the outline numbers, you will get a context menu on which one of the options is Font. Change the font from there, and it will be changed for the entire numbering for that outline level. This also works from Outline View.

Alternatively, you can click on one of the numbers or letters on the level you are interested in, then select the font from the drop-down Font box on the Home tab.

More useful Word 2010 help can be found at http://shaunakelly.com/word. Unfortunately the eponymous lady is now deceased, but her friends continue to maintain the site in her memory and because it is so useful. http://christinekent.blogspot.co.uk/ may also be helpful.

I realise this is probably a bit late for you, but I'm posting it for the record anyway in case it helps anyone else.