You can change the material referenced by a Waveform Region in a Cue.
From the context menu of the Waveform Region, select [Change Referenced Material Waveform...].

The "Reassign Material" dialog box will display a list of other waveform regions that refer to the same material.
Check the boxes to reassign as necessary.

Drag and drop the new material you want to assign to the appropriate location and click OK to apply it.