As noted above, new words to old tunes has a long and honorable history. The only catch is to make sure that the old tune is in the public domaine. At one point I was involved in a city centennial project and we had this guy who was mortally offended that we wouldn't use the songs he wrote about the city. Trouble was, he was using the tunes, and sometimes chunks of the words, of popular tunes about other cities...all modern and most definately under copyright.