I don't have an arranger keyboard so I'm not sure exactly how they are set up, but I thought something like this below, if you think that will work ?
If you hit a keyswitch that will be the next section played on a new chord or the same chord is played, the change will occur on next beat or bar. If no section keyswitch is hit it will continue with the current.

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BB21-Live-Arranger-GUI.png (163.99 KB, 408 downloads)