What about providing a new command `\upbeat` and moving `\partial` into oblivion? Compare this to `\tuplet` vs. `\times`. Werner... Or you could use the brand new command \upbeat when music follows, keep \partial, and you don't have to worry about backwards compatibility.
This makes a lot of sense IMHO since, linguistically, it's not the music that becomes "partial": it's the measure. The music becomes an upbeat (at least in most applications of \partial).
Lukas
