Dear Rabbi Kornfeld:
In our gemara regarding "hifuch" of spittle and the stira in our Mishna and the Mishna in Mes. Keilim, Rav Yochanan says 'Muchlefes ha'Shitah'. Rashi learns that Rebbi Yehudah "changed his mind" from the way he held in Mes. Keilim, wheren the spittle must be moved around in the mouth, to his opinion here in Eruvin, that it need not be moved around. Normally, "Muchlefes ha'Shita" means that we switch the mahn d'amrim. For example, if R Yehuda says "X" and R Meir says "Y", then "Muchlefes ha'Shita" means that R Yehuda says "Y" and R Meir said "X".
It seems from the Mishna in Mes. Keilim, the girsah is that R Yossi holds "hifich bah" and R Yehuda holds "m'taheir"; and no one seems to question or "correct" the girsa there. So it would seem that the "normal" meaning of muchlefes ha'Shita would fit just fine here. Why does Rashi have to tiecsh that R Yehuda "changed his mind" here?
Your question is very good. You are correct that "Muchlefes ha'Shitah" normally means that the opinions are switched around, and that is indeed the way RABEINU CHANANEL and TOSFOS (DH Hu Savar) here learn. There are a few places, though, where Rashi says that it means that the Tana changed his mind, and it seems that there is always a reason why Rashi does so.
Based on the Girsa of the Mishnah in Keilim as it is cited in our Gemara, Rashi did not want to say that "Muchlefes ha'Shitah" means that the opinions are switched around, for the following reason.
There are three levels of "Rok:" (1) when the salivary glands are working, producing saliva ("Rok"); (2) when a significant amount of saliva has collected in one's mouth ("Nitlash Roko"); (3) when one has prepared the saliva, by manipulating it with his tongue to the fore section of his mouth, to exit ("Hefach Bo").
In the Mishnah, Rebbi Yehudah says explicitly that "Nitlash Roko" (#2 above) is considered separate from the body, and therefore one may not even walk four Amos in Reshus ha'Rabim with it in his mouth. The Gemara adds that this is so even though level #3 has not yet been reached. In the Mishnah in Keilim, however, Rebbi Meir and Rebbi Yosi do not differentiate between any of the levels of Rok -- Rebbi Meir is Metamei even if the Rok has not yet been Nitlash (#1 above) and Rebbi Yosi is Metaher even if the person has been Hefach Bo (#3 above). If "Muchlefes ha'Shitah" tells us to switch one of those opinions with that of Rebbi Yehudah, then Rebbi Yehudah in the Mishnah in Keilim will still not be consistent with Rebbi Yehudah in our Mishnah! If we switch the Shitos and say there that Rebbi Yehudah is Metamei, that means even if the Rok has not yet been Nitlash, but in our Mishnah, Rebbi Yehudah requires the Rok to have been Nitlash in order to be considered separate from the body! And if the Gemara means to say that Rebbi Yehudah should be switched with Rebbi Yosi's opinion, that would also be inconsistent with our Mishnah because then Rebbi Yehudah would be saying that the Rok is not Metamei even after it has been Nitlash and even after Hefach Bo. It must be, therefore, that "Muchlefes ha'Shitah" means that Rebbi Yehudah changed his mind.
Rabeinu Chananel, on the other hand, who says that "Muchlefes ha'Shitah" means that Rebbi Yehudah is the one who holds the opinion expressed by Rebbi Meir -- that Rok is Metamei -- must understand that the Mishnah in Keilim is only referring to Rok after the stage of Nitlash, and not before. Rashi did not want to learn that way because the Mishnah in Keilim makes no mention of such specificity.
Sorry for the delay in getting back to you. All the best!