quotation item cannot be changed because of subsequent document (v1489)

Hello,

got a tricky situation:

- There's a quotation (quite a large one)

- parts of it are already delivered (including a Standard Order an Outbound Delivery and an Invoice).

- the invoice hasn't been brought to account.

That quotation has an item:

Position: 100
Material: X
Amount: 3

2 are already delivered so 1 remains. Now the customer wants the quotation once again but the left one shouldn't be material X, this time he wants material Y.

So the quotation should look like this:

Position: 100
Material: X
Amount: 2

Position: 110
Material: Y
Amount: 1

-

The questions:

How can we change position 100? The fields of the delivered items are all disabled and of course it's impossible to delete the line ("quotation item cannot be changed because of subsequent document", error code v1489).

Any suggestions?