The best way is to set a Position Sync on the currently playing stream to fire a few seconds from the end of the stream. When it fires call BASS_ChannelSlideAttributes() to begin sliding the current stream off, open the new stream, set the new stream's volumne to 0 and use the BASS_ChannelSlideAttributes routine to slide the volume up.
.Azoth.
I may have a sample project in VB somewhere if it will help.