apart from Premium Economy (if it's on your flight), what about purchasing a second seat (using that contract/discount fare) for medical reasons?

https://www.cathaypacific.com/cpa/en..._services_fees

Extra Seat For Personal Comfort or Medical Reason

The additional charge for a 2nd seat requested by a passenger must not be less than the applicable fare paid by the passenger for the 1st seat.

my guess is that purchasing a second seat would be cheaper than upgrading to business class.