Yesterday we mentioned that there was a unique heter for neveilah to be eaten, for treif utensils to be used, during the fourteen years that the wars of conquest for Eretz Yisrael were fought. The gemara (Chulin 17) raises the question of whether neveilah leftover from these fourteen years may be eaten after the wars have ended -- do we say that the food has a permanent status of heter, or was the issur of neveilah temporarily lifted for the period of battle and immediately went back into effect at the close of war. (As noted yesterday, the fact that the gemara even considers extending the heter beyond the close of battle seems problematic according to the Rambam's view that the heter is one of pikuach nefesh to avoid starvation.)
There are a minority of poskim who find in this gemara a solution to what to do with your leftover fleishigs from Shabbos once the 9 days starts. They compare kosher meat which was prepared before the 9 days to this case of neveilah which was permissable to be eaten during the years of battle -- in both cases once permissable, the status of heter remains; the food can be eaten even when circumstances change and new food of the same type would be prohibited.
The Chelkas Ya'akov (O.C. 214) writes that this comparison does not hold water. The analogy assumes that the gemara's safeik was whether or not to apply a new issur to food which previously had a status of heter. But that's not really the gemara's question. The C.Y. explains that even during the period of war, eiver min ha'chai could not be eaten -- the meat had to be killed in some way, just kosher shechita was not required. However it was done, the act of killing the animal served temporarily, during the years of battle, as a sufficient matir to permit eating any meat. The gemara's question is whether post-battle this same matir which was in place is sufficient, or whether the bar is raised even retroactively and only food shechted properly was permitted.
In other words, the issue is not whether a new issur applies retroactively to old food, but rather whether an old heter applies going forward even once the standards have changed.
Food prepared before the nine days did not have a "matir" that permitted it to be eaten -- there simply was no issur of eating meat. It is no analogous to the question of whether the matir of killing the animal in some way other than shechita extended beyond the end of the period of conquest.
The lomdus is nice, but the problem is the Rosh proves from the gemara in chulin that if a person takes a neder not to eat a certain type of food, he can finish leftovers of that food that he had before taking the neder. The case of neder clearly seems parallel to the issue of food during the nine days. (See Aruch haShulchan 551:24 who. among others, addresses this issue. I think perhaps you could distinguish between neder and neveilah, which are issurei achilah, and not eating meat during the nine days, which is a kiyum mitzvah of aveilus, not an issur achilah on the meat itself.)