The following regression corrected recently in XnView MP 0.98.2 seems to be present in XnConvert 1.90.0 :
Transparent PNG modified when 'Change color depth' selected
The bug also seems to be present in XnView MP 0.98.2 Batch convert...
Transparent PNG modified when 'Change color depth' selected
Moderators: XnTriq, helmut, xnview