I think the way to save future effort in maintaining a website (especially a large one) is to do a fair bit of forward planning.
I'm not sure how much of your site is to be redone, or how much is 'finished' as far as content is concerned.
Normally, one would create a page 'type' (i.e. 'Home Page' or 'Content Page') and include in that page all the elements that are going to be the same for duplicated pages (divs, header, footer, side columns etc.)
When the page type design has been finished (including a site menu) then that page would be saved as a template. You could end up with a template for each page 'type' that you foresee being used in the site in the future.
So if I had a Page (say a 'content' page) had added the header, the footer, a div for the navigation and the menu itself and the divs (or table) that form the layout style I want, I would go to the Dreamweaver menu and select 'File' > 'Save as Template', and when prompted to 'Update links?' I would click yes. This template would be automatically saved in a folder called 'Templates' in the site's root directory.
I would then, with the template open in Dreamweaver, select the various areas that I wanted to be 'Editable Regions' on any new page created from that template, and for each one selected, I would again go to the Dreamweaver menu and select 'Insert' > 'Template Objects' > 'Editable region', name the region and save the template. I would not include Header, Footer, Navbar or the Menu itself as editable regions, because they would (normally) be universally the same throughout the site.
The next step would be to create all the new pages from that template: (Dreamweaver Menu > File > New > Page from Template (select my template). All new pages would be created this way (from a template).
This is where your query gets answered:
With the site open in Dreamweaver, if I wanted to change something in the template in a 'non-created' editable region (i.e. a menu item) I would make the change and save the Template file. A dialogue box would then ask me if I wanted to update all the pages based on that template, and I would just need to click 'OK' for that change to be applied to all such pages, whether it be ten or a thousand - the pages don't need to be open, just existing in the Dreamweaver site definition, as created. Remember also, that you can have more than one such template for more that one page 'type'.
If, when you perfect your menu, you are able to do things this way, then I strongly recommend this method. It could save a lot of time and worry in the future.
There is another way that happens more-or-less automatically if you have a testing server set up on your local machine (ie XAMPP MAMP WAMPserver, IIS7) and your pages are all created as .php pages, but I won't go into that just now unless you need more info about it.