Lecha Dodi - Changing the Tune
There is a widespread custom to change the tune of "Lecha Dodi" when reaching the stanza of "Lo Tevoshi." It is quite unclear, however, where exactly this custom originates from. Some sources even suggest that the practice is baseless, without reason, and possibly even in error.
Among the earliest sources which discuss a custom of changing the tune during Lecha Dodi is from the community of Frankfurt-am-Main. It is reported that in Frankfurt-am-Main the custom was to change the tune of Lecha Dodi at the stanza of "Hitoreri". However, when reaching the stanza of "Boi B'shalom" the congregation would then revert back to the original tune which had been used from the start of Lecha Dodi until having reached "Hitoreri."
It is explained that the reason for this custom was in order to emphasize the first four stanzas of Lecha Dodi. This is because the first letter of each of these stanzas is an acronym for the word "Shlomo" which is the name of the author of Lecha Dodi (Rabbi Shlomo Halevi Alkabetz). It also serves to remind us that mashiach will be a descendant of King David through his son Shlomo. Furthermore, since the first four stanzas represent exile and the last four represent redemption, it was decided to sing each of these themes with separate tunes. It is also noted that the first letter of each of the last four stanzas represents the second word of the author's name, "Halevi," making it an appropriate time to switch tunes, as well. As the final stanza of "Boi B'shalom" is unrelated to either group of stanzas, it was decided to revert back to the original tune with which Lecha Dodi had begun.
There is a widely cited theory that the change of tune at "Lo Tevoshi" is intended to correspond to a switch in the theme of the Lecha Dodi from that point onwards. This, however, is simply incorrect, as the change of theme from that of exile to that of redemption actually occurs in the stanza prior, the stanza of "Hitoreri,"as mentioned above. This strengthens the logic and legitimacy of changing the tune when reaching "Hitoreri" rather than when reaching "Lo Tevoshi."
It is also suggested that the change of tune during the course of Lecha Dodi was simply in order to prompt those in the congregation who may have lost their place as to where the service was now holding. There is also a theory that the custom of changing the tune specifically at "Lo Tevoshi" is related to the fact that it is the first and only stanza of Lecha Dodi to be worded in a negative manner ("lo"). Alternatively, it might just be that since the entire kabbalat Shabbat service is somewhat new and essentially optional in nature, chazzanim took advantage and used the opportunity to apply their own musical creativity and additions.
There have been many other customs throughout the centuries regarding the manner of singing Lecha Dodi, as well. For example, in Breslov the tune was changed when reaching the stanza of "V'hayu Limshisa." In some communities every stanza of Lecha Dodi was sung in a different tune and in others the tune was changed every two or three stanzas. It is even reported that the Cantor R' Aharon Ber of Berlin composed a different tune for singing Lecha Dodi for every Shabbat of the year! In Kahal Adat Yeshurun during the Three Weeks the tune of Lecha Dodi is changed at "Hitoreri" from minor to major - then back to the original minor tune again at "Bo'i B'shalom."
It is also interesting to note that it was primarily in the Polish and Galician Chassidic communities (Gur, Alexander, Belz, Tzanz, Modzitz) that Lecha Dodi was even sung at all, while in many Ukrainian and Russian Chassidic communities (Chernobyl, Kassov, Karlin, Lubavitch) it was just chanted without any special tune or harmony. In Kretchniff it was sung to the tune of Akdamut.
 Seder Kabbalat Shabbat (Cohen, Yosef Y) p. 47-59 cited in: www.biu.ac.il/JH/Parasha/teruma/kle.html
 Cited in Likrat Kalla p.147.
 Siddur Mekor Hatefilla p .202 cited in: www.biu.ac.il/JH/Parasha/teruma/kle.html
 Likrat Kalla p.261