Someone I have linked to before, (Serdar Yegulalp) has another very useful posting for those who are thinking of mass-migration of documents over to  Highly recommended.

Question though, if you already have a large installation of MS Office formats, why not just leave the default document format for everyone as the latest version of the MS format?  You can choose the default format in OO.o very easily, it’ll make saving documents in that chosen format a piece of cake, no worries and costs for mass-migrations needed.

Read the article, think it over.