OK it works with Premium Edition.
One thing that doesn't work though is 'type' => '*' in order to apply the new font to all the document.
The following doesn't work:
$docx->customizeWordContent(['target' => 'document', 'type' => '*', 'contains' => '',], ['font' => 'Calibri']);
So I have to loop throught all possible types, like this:
foreach (array('break', 'image', 'list', 'paragraph', 'run', 'section', 'style', 'table', 'table-row', 'table-cell') as $type)
$docx->customizeWordContent(['target' => 'document', 'type' => $type, 'contains' => '',], ['font' => 'Calibri']);