UPDATE - Here is the email I received from the Pet Food Institute regarding the six month window for label changes:
"Most ingredient
changes are very minor, and if so, a short allowance is made for using up
packaging. MAJOR changes require that labels be changed As Soon As
Practical. It takes several weeks to get packaging changed, depending on
the difference in the label. If it is just an ingredient, itself, that is
a much simpler change than if you’re making a statement about that
ingredient (“Calling it out”). Basically there’s about
a 6 month allowance to get ALL packaging changed, because of distribution and logistics
issues.
Significant label changes – those dealing
with the first 6 or so ingredients have a much higher threshold to meet before
labeling can be changed, with formulations, palatability challenges,
digestibility challenges, etc., so ingredient changes are pretty rare in the
grand scope of pet food."