If you make changes to a template after an order is placed, and then a customer places a reorder of the product that uses that template, the updated template will be used, instead of the original one. This is the case whether the customer edits the document or not during their reorder.
Please note: If you have added a field to the template's master PDF between the time the customer placed the original order and the time they attempt to place a reorder, they will receive an error. It is necessary for them to begin a new order if you have added a new field. Also, if you assign a new template to a product, they will not be able to re-order.
In cases where you change something in the Field Setup tab, the old values may not work. If you are changing the settings (text formatting, spacing, tracking, etc.) then you may want to give them a new template to be safe. A new template would not allow them to re-order but you will be more certain that the template is being completed correctly. If you decide not to create a new template then check over any re-orders carefully.