How can I run templates on existing issues?

Answer:

To run templates on issues after they are created, simply follow these steps:

Create a new plan in ASP
Add parent issue type that you want to normalise
Add the subtasks that you want created for these existing issues
Add a condition to this plan / parent issue type that looks like the following:
Save the condition and the template
Make sure that the project(s) in question are included in this plan
In your jira, search for all the issues that you want retroactively affected by the plan
Bulk edit the issues on the list to add the label "legacy_tickets"
Confirm bulk edit

All these tickets will be affected by the new plan and therefore, all will have the listed subtasks added to them