A common scenario that ArcObjects developers come across is creating a button or tool in ArcMap whose image changes when clicked.
Example: Say you wish to create a “LogOn” button whose bitmap needs to change to “LogOff” bitmap once the user has logged in.
Just setting base.m_bitmap to the new bitmap does not update the bitmap in ArcMap. Instead you need to use the BaseCommand.UpdateBitmap method.
Per ESRI .Net documentation:
UpdateBitmap changes the Bitmap for the command. Call this method to update the icon used for the command at runtime, after the command has already been instantiated.