Say I've got Mix A already imported and added to a Layer. I later get Mix B and want to replace Mix A without changing any labels or volume automation. After importing Mix B as an Asset, I can do this one of two ways:
1. Open the SmartEdit popover in the Waveform view and click the 'Content' tab.
2. In the Asset dropdown, select the respective channel of the Mix B Asset, Left in this case. Do the same for the Right channel SmartEdit.
3. Open the SmartEdits pane and add 'Asset' to the column display options. I find this method easiest, because you can enable 'Edit All Channels' and have both channels replaced at once.
4. Select Mix B from the newly added 'Asset' column.