What condition is the cordyline in, overall? If it's growing normally and the yellow leaves are at the lower part of the stem then it sounds like a simple matter of a tidy up....just cut or peel (if they're loose) the discoloured leaves off. I've just done the same to mine and it's perfectly normal.
But as I said before, it depends what portion of the plant is yellow because it's a different story if the whole thing's gone yellow!