I'd like to see two changes to the invoice form:
1. allow changes to the item. the best way to do this would be to allow a pull-down so that the membership level (or whatever) can be changed. People do make those changes and this allows the change to be made easily and to have it connected to the financial transactions. Without this option, my experience has been that it's a convoluted process to change their membership level.
2. insert a quantity. Sometimes people want to renew for more than one year, and the invoice does not allow this.
How do you feel about these ideas?
Automatically generated invoices are tightly connected to the operation that created them - so changing them would not be an easy and even proper thing to do.
You can delete the invoice (or create a payment with some special tender in it, like "credit note"), update the membership and then generate the new invoice using system suggested buttons in the contact profile. Would it work for you?