You have a book that is composed in one language, and you want to "convert" it to another (for example, you want to change your U.S. English to U.K. English).
The long solution is to go through each paragraph tag and change the language, but there are a couple of shorter solutions.
One is to load your docs on a machine that has the dictionary you want to use, but not the dictionary you are presently using, and whose default language is the new language. You will then get messages about unavailable languages, which will be substituted with the default language.
Another is to save the files as MIF, then replace the string "USEnglish" with "UKEnglish".