Bottom line is that UA's COC is very clear. There is no right to a specific aircraft or soft product. That includes lie flat, IFE, meals and the like.

When UA first introduced the 787 and the aircraft was grounded, it voluntarily agreed to refund/change for those who asked. UA made it clear that this was a one off as there were many who paid simply to fly a short domestic hop on that aircraft.

Make your best case when you call. Now that UA does a better (?) job of documenting PNR notes, it becomes less and less likely that one agent overrides the one before.

If there is a substantial schedule change, that is a different story and has nothing to do with OP's question.