Hi Morton,
What you are trying to do should work BUT it dosen't at the moment due to a fault with the program.
The way to do what you want for the moment is to "unfold" the song.
Go to the EDIT menu SONG FORM then UNFOLD (Convert to 1 Big Chorus)

Your song will then expand to 1 large chorus not 5. you can then use the Bar Settings (F5) at the bar for the chorus you want to change tempo at and do it there, then go on to where you want to go back to the original tempo and set the tempo back there.
Col

iMac, 3.7 GHz 6-Core Intel Core i5, BIAB MAC2024 (802), Audient iD4