Winnie, Chazal did not initiate the observance of Lag b’Omer. In fact, the entire partial mourning of the Sefira is a minhag. The Chatam Sofer, in fact, objected to it (http://www.hebrewbooks.org/pdfpager.aspx?req=14664&st=&pgnum=193 continuing to the next page) as no miracle happened and onme davka should mourn Rashbi’s death (although, actually there is no evidence for this – see “A Printing Mistake and the Mysterious Origins of Rashbi’s Yahrzeit” by Eliezer Brodt). However, there is an obligation to establish a day a simcha and hoda’ah for deliverence of Am Yisrael (http://hebrewbooks.org/pdfpager.aspx?req=794&st=&pgnum=146) – such as Yom HaAtzmaut.
As for notchanging, Chazal did make Purim Meshulash when Shushan Purim falls on Shabbat. In fact, in their time there were several days on which the Megilla could be read (Mishna Megillah 1:1).