When I've combined threads, I post this notice in the first message of the thread:
Messages from multiple threads combined. -Joe Offer-
Sometimes, when I see a need, I do put markers like this:
Messages below are from a new thread.
Maybe "later thread" would be a better identifier. Note that the posting dates and the original message title remain as originally posted - so in some places, you can see whey I've combined lyr req, lyr add, tune req, etc...
If I've moved all the messages and closed a thread, all the links pointing to that thread end up dead. That's a disadvantage, but not a horrible one.
The Donegal Danny thread is a good example of what happens whn I combine. There were about eight threads on this song, and some of the threads contained only two or three messages. Combining them gives the thread a bit of a hackneyed appearance - but at least, all the information on the song can be found in the same place. If I contintinue to combine threads in the future, I think that a lot of the duplication within this thread will be eliminated.
In situations where a person has a new question about a song or subject, I think the practice of combining threads may help encourage people to title and phrase they posts so that it's obvious that this is a new question. I helped somebody along with that in this thread (click), which had been titled "The Water is Wide - one more time." I combined it with the other "Water Is Wide" threads, but then somebody pointed out that this person was looking for the first American version. So, I moved the request back into its own thread, and titled it "Water Is Wide - First American Version." Maybe when the discussion has died out, I can combine or crosslink it with the other threads on the song.
I usually don't do anything with "BS" threads except make sure that they bear the title "BS." I will, however, combine threads if there is more than one running on any give subject.
When I do editing of music threads, I think you'll notice that I make it quite obvious - and I take special care not to delete any information that might be of use to somebody.