Hello
Just want to update that in latest version of Membership Pro, it is simpler to change the output of certain field types using override. You just need to modify code components/com_osmembership/view/fieldlayout/fieldvalue.php
In that layout file, you can access to 3 variables
$row : The custom field object
$rawValue : The raw value of the field (what's stored in database)
$fieldValue : The current formatted value
You can just override that single file and the change will be applied for every places.
Regards,
Tuan