If I create a new PDF document with a series of form fields and then need to update, e.g. the word document is changed to include or change fields, can I update the existing PDF without having to re-do all the fields
Yes you can change the word document, then make a new PDF then replace the form with the fields with the new PDF.
If you are careful and do not change the position of the fields then it will fit right in.
If you change or add fields then with Acrobat you will have to rearrange the fields to match.
The easy way is to create a PDF from the updated word document, open up your PDF form, then replace pages in the form with the pages from the new PDF.
Your form fields, as well as their respective positions, will be retained, as will your scripts.