How to modify an existing order using a cart page

The client wants to be able to go to an existing order, and change a quantity or price or something, and then save it that way.

I have it working so that they can look at all the orders for a given customer, and go in to a specific order and look at it in a cart page, and then reorder it if they want to with changes.... but the client wants to be able to change that order without re-ordering it....just saving it with the changes.

Since that cart page is showing info from both the orders table and the orderdetails table... I assumed you could add a dataassist behavior to store cart details in database, and store cart summary in database when they clicked on a new button... but then I realized that it would make a new record, instead of updating the existing one...right?

Then I thought of using the multiple records update...but when I tried it came up with an error saying I had to have a repeating form area on my page...which I do --- its the cart page... but it thinks I dont.

Anyway... am I on the right track? Originally I told them that they probably would have to delete the order and redo it, but of course they dont want to do that. I also figured I could make a form that edits the record, but I would have to edit the order and the details, and then do the math, etc....

Aghhh. I am confused....so maybe someone can help. thanks.