2- Yes there is. The ships are still on the c-store and I bet you they have not been selling well (if at all) because the newer ships outperform them by orders of magnitude. Why would you buy a Nebula when you can buy a Vesta?
No, there isn't. Why would they upgrade a ship that already exists, that people already have, when they could add a new one that everyone would have to buy anew?
If they wanted people to buy Nebulas again, they'd put up a new, improved variant, not upgrade the old one.