A major shipping company proposes a series of modifications to the property assignment contract. Is this possible? is there another type of contract?
Usually each company daft-up legal documents that suits their legal requirements. I take it that is the case for Adobe. However if an amendment is necessary for you to secure the signature for the photos, you could draft-up a release, but be sure to include all the requirements of Adobe along with the shipping companies suggestions. After you are done you may message directly to Adobe via "Contact us" at the bottom of your contributor page, with the amended document asking them to take a look, and to advise you how to go ahead with this document you have drafted. Adobe's approval is crucial since it is important that the new document protects you and Adobe, and not restricting your legal rights as the owner of the work. I know it can work, since I know of at least one other person using their own drafted document, but I am not sure of the procedure.