No, there isn't. Unless there's a schedule change or some other IROP that allows you to change the flights for free UA will need to charge you the $150 change fee (or more if it's an international flight outside of the US/Canada).

Remember, fares can be different on codeshare flights because AC may have sold all of the seats they want in the lower fare buckets on their flight but UA may have extra inventory allotted to them to sell into the lower buckets.

-RM