Frequently, perhaps even most of the time, I've noticed that the chazan (prayer leader) for Kabalat Shabbat switches tunes for the last four verses of Lecha Dodi. Is there any reason for this, or are people just doing it because other people do it?
There is an old minhag Frankfurt to sing the first part of Lekha Dodi, which refers to exile, to a slow tune, and then switching to a happier nigun when switching to nechamah (consolation) about the future redemption at Lo Seivoshi. However, in that minhag, they switched back to the original tune for the last verse, Bo'i Beshalom, which -- like the opening verses -- refer to Shabbos, not redemption. (See Liqrat Kallah pg 147.)
According to R' Ari Enkin, writing on Hirhurim:
Although there is a widespread custom to change the tune during Lecha Dodi when reaching the stanza of "Lo Tevoshi", it is actually quite unclear where this custom derives from. Some sources even suggest that it is baseless and evolved without reason.
I recommend that you read the whole piece and see what he's uncovered. Indeed, there doesn't seem to be a good reason to change tunes then. If anything, it might make sense to change tunes one verse earlier.
I have often been quite thankful for this minhag (although I don't know the source). There are many times when the chazan (prayer leader) will choose a tune which is (a) painfully slow or unmelodic, or (b) nobody in the congregation knows, leaving him to sing alone. The tradition of switching tunes halfway through provides an instituted opportunity to cut the bad tune off and try again.
This is especially helpful for lecha dodi where there is such a large number of tunes that exist.