Hi thanks but I don't have to change the "tempo" but only the "feeling" in 2 or 4 (it's called double time). The "tempo" of the bar is always the same (does not change). It change only the time internal the beat (that sounds in 8 for example and not in 4, but the metronome is always at the seme value). In Jazz double time is very common in ballad (for example).
Another information. I use realtracks not midi. BIAB has a specific function (double time triple time half time) but not for a small group of measures of only for a part of the structure.
I don't know if now it is more clear.
Thanks
Toni