A few days ago I ran across a situation where I had to transpose a piece from C to Eb. Every time I tried using the transpose command, all it did was leave the key in C but added all the flats throughout the piece rather than change the signature to 3 flats. I found that if I added a G-natural signature to the C version and then did the transposition, it worked automatically. Can anyone tell me why this is? I was quite surprised to say the least.
Normally you would place a "C" key signature before transposing if the piece is in "C". This results in a hidden natural sign on the "F" line.
I don't fully understand the reason for this quirk but it is well known in the community and comes up semi-regularly.
See also:
FAQ - Transposing a staff
http://ntworthy.com/composer/faq/95.htm