In Bamidbar 31:23 the posuk teaches us that every vessel that was used with fire shall be koshered with fire and every vessel used with hot water requires to be koshered with hot water. The first question we need answered is; how can we kasher keilim? We have a rule “ein mevatlin issur Lekatchilah.” How can one be mevatel the issur of the goy’s kli by koshering it? The Shailos Utshuvos Rashba vol 1:495 among others, answers that the rule stating we don’t allow one to be mevatel an issur applies only if after you are being mevatel the issur you end up eating that issur in its nullified state. In our case, we are squeezing out the issur from the pot and getting rid of it. In that case it would be allowed; otherwise, you would never be able to kasher a treife pot.
Another question that we need an answer to is why don’t we make a bracha when we kasher kailim the same way we make a bracha on tevillas keilim from a goy? The Meiri Megilah 21b says that toiveling a keli is a positive mitzvah; therefore we make a bracha. Koshering a keli is not a positive mitzvah, but rather a way to avoid being oveir many lavin, therefore we do not make a bracha.
The kler that we can try to analyze is whether one may take a kli that was used for meat and kasher it to use for milk or vice versa? Shulchan Aruch Orach Chaim 509:5 says one can be melaben (burn directly on the fire) a metal keli that was used for dairy and then make kugel with meat on it. The Magen Avraham writes that from this we see we may kasher from one to the other, but the oilam is noheg that they will not kasher from one to the other. The reason is brought from Reb Binyamin of Pozna that if we allow this to happen, everyone would have only one dish and constantly switch it by koshering, and mistakes will be bound to happen.
The Shaar Hamelech in Hilchos Yom Tov 4:8 asks that based on the Smak that differentiates between libun (burning in the fire) and hagalah, (using hot water) libun may be done to a kli that is a ben yomo, whereas hagalah may not. There would be an allowance for libun to be done from milchigs to fleishigs or vice versa, but not for hagalah to be done.
The minhag is not to kasher from one to the other. When a person has a great need, kashering may be done in the following circumstances. If one is koshering a kli for Pesach, then the kli may be switched to fleishig or milchig. Since his kavana was not to kasher it for the switch, but rather to kasher for Pesach, it is therefore okay based on the Teshuvos Chasam Sofer Yoreh Deah 110. A second circumstance would be if one makes the kli that he wants to switch treife, he now has to kasher it, so he is not koshering it with the intent to switch, but rather to make the kli kosher again. The Mishna Berura Orach Chaim 509:25 brings down you would then be allowed to switch. Some say that this is just a minhag and therefore one can kasher it from milchig to fleishig or vice versa, based on the Pri Megadim 451:13. Another way of doing it based on the Maharsham 2:241would be to kasher a meat pot and then use it for pareve for a while. At a later point, switch the pot to milchig. The final way is also based on the Maharsham 2:241, which is if the pot has not been used in 12 months, then you can kasher it and switch it.
May we be zocheh to keep Kashrus and tevillas keilim properly!