It is important to understand that once a template has been used to create the first "real" document, you should refrain from making ANY updates to the template. This is because the S-Docs template record is used both during document creation AND during each subsequent view of an S-Doc. Each S-Doc stores the template used to create it. If you attempt to open an S-Doc where the template used to create it has been deleted, you will receive an error. The diagram below illustrates this relationship.
To edit templates without affecting previously created documents, you can use the Auto Create Salesforce Attachment and link to record option (found under the "Document Options" tab in the template editor, this feature is enabled by default). This feature creates an attachment each time a document is generated and stores it in the "Notes and Attachments" related list on the base object. Attachments allow you to store a copy of the document that cannot be affected by template changes and do not require the S-Docs template record to view. This is particularly useful for orgs that are restricted to 2 templates, have a need to change templates often, and still require past documents to remain unaffected by future template changes.
If you don't use the Auto Create Salesforce Attachment feature and want to make changes after you have completed your design work, it will affect how PREVIOUSLY generated documents are rendered. Therefore, if you need to make changes, you should CLONE the template and make modifications to the newly cloned record.
You then uncheck Available for use and Initially Visible on the original template to ensure no new document can be created using it.
Previously generated documents will continue to use the original template (therefore rendering unchanged), and new documents will use the newly activated template.