I obviously can't comment about what is "original" or not, because we don't know what the "original" wording was, if there ever was such a thing. Chazal left us nothing of the sort.
What we do know is למשומדים is found in just about every surviving manuscript, from Yemen, to Ashkenaz, to Spain. The major article on the subject which you'll want to read is here. It's found in the Seder Rav Amram Gaon, Seder Rav Sa'adya Gaon, Machzor Vitri, Ri ben Yakar, Rambam, Avudarham, Rokeich, Rosh, plenty of other Rishonim, Tur, Kesef Mishna, as well as basically all the Siddurim we have from those times.
If you look some of those up in your printed editions you might not see למשומדים, but that just means your edition copied from a censored edition. The censors were often themselves apostates (ie. משומדים) and hence really worked to get rid of this word. We have old manuscripts from all of those listed above with למשומדים. As an example, here is the Rosh referring to the blessing by the title למשומדים, and here 80 years later it's "called" למומרים. Here is an early Tur with the word משומד, and here is a later edition with it literally just crossed out. Many times they'd change it to כופר, מומר, רשע or something like that. Go look on your shelf to see what made it to your edition.
R' Meir Melamed about 400 years ago (Shu"t Mishpat Tzedek 2:9) notes that some locations near him started printed למלשינים instead of למשומדים to avoid the eyes of the censors, but would still say it correctly in prayers (like a Keri/Ketiv). He was asked if a Chazzan who actually said למשלינים aloud needs to be removed as a suspected heretic, and is unsure but leans towards yes if he refuses to recite the traditional version even when asked to. This is quoted by the Keneset HaGedola who in turn is quoted by the Magen Avraham (126:1). The Magen Avraham, also quoting the Keneset HaGedola, further notes one community which, due to fear (Machatzit HaShekel), actually was forced to start saying למלשינים in prayers! Later authorities (see Shulchan Arukh haRav and Mishna Berura) rule that therefore the Mishpat Tzedek's question is now able to be answered in the negative because it doesn't look problematic to recite the fake version.
It should be noted the Magen Avraham too was censored (לכופרים is what you usually see printed there in current editions, which confused many later authorities such as Magen Gibborim and Arukh haShulchan, though Mekor Chayim (Bakhrakh) correctly understood the subtext) and even R Yaakov Emden in the 18th century still felt unable to print the old wording (though he noted it in his Luach Eresh).
Eventually nearly everyone forgot. Sad :(