I have already figured this out, so will explain it here.

However, it took time trying to figure it out, and it works in a very un-intuitive way, so I hope this function will be improved.

First of all, you may think you are setting the original tempo of the piece by typing it into the tempo box. One would think so. However, that only works if there are no tempo changes. So highlight bar 1, right-click or edit menu-bar settings, and put in a tempo change of your original tempo, although you are not changing anything.

To do what I was trying to do--go to the measure you wish to make the change. First choose 1st chorus (before typing in a tempo), and type in the original tempo again, then choose 2nd chorus and type in the changed tempo for the second chorus. Even for the "change" on bar 1 (setting the original tempo), to be safer, I might suggest rather than leaving the default "all choruses", set to 1st chorus, "change" to original tempo, then set to 2nd chorus, and enter the original tempo again. In other words, in order to enter one tempo change, I had to enter four, three of which were not a change at all, but the original tempo.

This certainly should be made simpler and more intuitive.

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Furthermore, I will add the following, recently discussed in another thread. It would be nice if one could do real ritardandos and accelerandos in BIAB, that work with Real Tracks too, not only changes at the bar line. Like the "fill" function in RB, or "ïnsert series of tempos" in Sonar. One could open the BB file in RealBand, and do it there. I tried it, and it does slow down the realtrack too, but the sound quality gets very bad, with added noise, etc. It would be nice if that could get fixed to work well, and in BIAB as well.