How certain are you of this advice? As OP sets forth the facts, AMEX CORPORATE is his employer's TA and they have certain policies they follow. This is generally because the employer has a negotiated contract, in this case with UA, which provides a discount. Those contracts require that an enrolled TA handle the arrangements, changes and the like (except of course IRROPS enroute).

1. Is it possible for UA to pull the ticket? Ultimately yes. But, UA CS can't reissue corporate discounts so when the ticket goes back through audit, the TA gets a DM for the balance between the discounted and general public fare. OP's employer foots that bill.

2. Is it possible for AMEX CORP to make changes, including upfare to C? Of course. But, AMEX has a contract with OP's employer and is obligated to follow the corporate contract. Here, OP's employer doesn't allow OP to purchase a C ticket from AMEX CORP and only be reimbursed at Y.

In the end, there are a lot of messy back office consequences. When it's an individual purchase or the employer doesn't care, it doesn't matter. But, where as here, the employer is clearly persnicketey for reasons we don't know, OP needs to start with his travel executives to determine how they want this handled. A 20% discount on a USA-Asia ticket, even in Y, can be a hunk of money and not exacrtly a career-advancing move for OP if he steps on a lot of toes.