No-one is quite sure when and how the words got changed to Happy Birthday, but the lyrics appeared in a songbook in 1922 and instructions were given as to how to insert the child’s name into the third line. At this early time, there was no record made of a copyright or credit to the song’s writer.
In the 1930s several films and Broadway shows used the song and the Hill sisters were not compensated for their work. A third sister, Jessica, filed a lawsuit to prove that 'Happy Birthday to You' (as it was called in the beginning), was their song but with different lyrics. The court took the side of the Hill sisters and from then on they were paid each time the song was used in films, shows or radio. The Clayton F. Summy Company, with the assistance of Jessie Hill published and copyrighted the song.
Many years went by, then in 1988, Warner/Chappell Music purchased the company that owned the rights to 'Happy Birthday' for somewhere in the region of 55 million US dollars. This copyright is said to run out in the year 2030, in America although in the EU, it is set to run out at the end of 2016.
Technically, all this means is that anytime anyone sings 'Happy Birthday', royalty payments should be paid to Warner/Chappell. It can’t be too difficult to collect royalties when 'Happy Birthday' is used in the media, but there’s obviously no chance at all of any money being collected every time it is sung at a child’s birthday party. I wonder how many times a day it is sung throughout the world?