Any custom templates you created for your customer have been added to a customer group. The group will have been named after your customer and your customer will be assigned to that group. The system will continue to send emails to your customer using the custom template you originally created, unless you decide to move your customer to a new group.
If you click on the Options section under one of your customer profiles, you'll see your customer is now assigned to a group and you'll still be able to customise the templates from this area:
You can also navigate to Credit Control > Settings to view your customer groups. If you selected a specific group, you'll be shown all the current templates, sending schedules and settings for this specific group with the ability to amend them: