If Lyrics Are Added Later ...

Dear Rich: I purchased and enjoyed reading your book "Getting Permission," but I can't find an answer to the following question: I am a lyricist, and a composer has asked me to write lyrics for his songs with the intention that we would each own 50% of the copyright in the resulting song (music + lyrics). I have done this with other composers and registered the song (music + lyrics) copyright in our names jointly. However, this composer wrote some of his songs years ago and already registered the copyright on the instrumental composition solely in his own name. For these songs, what's the best way for us to end up jointly owning the music + lyrics 50-50? I wondered if we need to sign an assignment agreement whereby he assigns 50% of his ownership in the existing music composition to me (and we'd register that assignment)....and then we'd file a registration for the song like usual where we each own 50% of the music + lyrics? I don't want him to continue to own 100% of the existing instrumental composition - I want to own half of that, just as he will own half of my lyrics. If you want to be a half owner of the instrumental, you will need an assignment of 50% of the copyright to the song.
Are you joint authors of the musical work? As for the version of the song with the lyrics, you can register this as a jointly-authored work provided that each of you prepared your contribution with the intention that it would be eventually combined (into a single song). Even if the composer didn't know you at the time of writing the instrumental, it can still qualify as a jointly authored work if the composer had always intended that words would be added. In that case, you can register the song as a jointly-authored musical composition. The law presumes that your shares are equal but its best to memorialize that arrangement with a collaboration agreement. Otherwise, confusion may occur.
If the composer did not originally intend to add lyrics ... In that case you can register the words and music as a musical composition as a derivative version of the original instrumental and list yourself as a co-owner (assuming composer assigns you a 50% ownersip interest).
P.S. Thanks for using Getting Permission, but we think Music Law might provide more help.