Songtrix

You can switch between substyles at different bars by using Song-Marker (although the many of the rough samples styles currently included with the beta version don't have different substyles yet).

Another way is to render the autostyle events.  Lets say you have an 8-bar song, you want bars 1-4 to be Dixieland and bars 5-8 to be Dance 1 for the Bass track.

Create a new Autostyle song 8 bars long in Dixieland style.  The Bass track should be a single autostyle segment.

Select bar 5 on the Bass track, and use Segment-Split to break it in half.  

Select the first segment (somewhere in bars 1-4) and use Segment-Autostyle to remove the autostyle setting from that segment.  The autostyle events now become "real" events.

Now change the style to Dance 1, and only bars 5-8 are affected by the new style.

Note that the events are separate for each segment, but the whole track is affected when the Voice or other Track Setup options are updated.