Thanks for your message. I think I know what you are referring. In the past, our Bitnami Cloud Hosting machines allowed you to easily "replace" Apache with NGINX by doing the steps you mentioned. But unfortunately, it is no longer possible. Can you share with me the link to the guide you are using so I can report it to the docs team for them to update it?
Regarding your current server, please note installing the Bitnami Drupal stack will always install Apache and configure Drupal to be served using it. Installing the NGINX server from the system packages and configuring it to server the Drupal application installed in the
/opt/bitnami/drupal directory is something we can't help you with, as we don't support it.
What we recommend is to create a new Bitnami NGINX machine and manually install and configure Drupal as explained in the guide I shared. You can migrate your current Drupal info using the official tools for this. I found a module related that I think may help you
We also recommend this way of proceeding to avoid having an extra Apache service on your current service that, although you can stop, will be consuming disk space.