Workflow Templates can be edited at any time (add or remove tasks, change due date calculations, etc.).

However, when a workflow template is edited, the newly edited version won't have any impact on the cases that the old template version was applied to. Learn more about editing workflow templates.
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The process is straightforward. However, if you have edited existing workflow templates that have already been applied to a case, it's important to understand what will happen to cases in which the old workflow template version was applied to. Namely, the due date calculation component.

One important component of workflow templates are the date calculations they use (learn about date calculations here). When you apply a workflow template to a case and that template has items with date calculations on it (tasks and/or events), you will need to specify the calendar dates used to calculate the task and event dates.

After a workflow template has been applied to a case and the dates are applied, you can always go back to the case itself and edit the calendar dates used for the due date calculations.