When you edit an existing event or booking template, use Save as new template to fork your changes without changing the original.
Event templates
Open a template from Settings → Event Options.
Click Save template, then the caret → Save as new template.
Name the new template and save.
Booking templates
Same pattern from Settings → Booking Options or from an event template’s Bookings tab.
Duplicate vs save as new
Duplicate in the templates list copies a template you are not editing. Save as new saves changes from the open editor as a new template.