Select the Characters -> &htmlchar; command in the Convert menu to replace all characters that have HTML entities with those HTML entities. It replaces "après-ski" with "après-ski". If you select some text before using this command, only characters in the selection are replaced. Otherwise, all characters in the file that have HTML entities are replaced.
Characters that do not have HTML entities are not changed. If you want to convert your file into pure ASCII using HTML entities and numeric character references for non-ASCII characters, first use the Characters -> &htmlchar; command to replace those characters that have named HTML entities. Then use the Characters ->  or Characters ->  command to replace the remaining non-ASCII characters that do not have named HTML entities.
This command does not replace reserved characters such as <. Thus you can safely use this command on an entire HTML file without destroying its HTML tags. If you do want to replace reserved characters such as < with entities such as <, use the Convert|Reserved Characters -> XML Entities menu item.
Convert| and  -> Characters
Convert|&htmlchar; -> Characters
Convert|Reserved Characters -> XML Entities
Convert|XML Entities -> Reserved Characters