Originally Posted by
onlySIA
Not strictly true, for some routes you can force a mixed class booking by selecting flight combinations where one flight is on the A350 or other plane that doesn't have F. And you can directly book a round trip with outbound in J and return in F, or vice versa.
Yes you can combine outbound J and return F or vice versa. But ops question was not about outbound J and return F as his trip was in one direction.
In his case where one leg doesn’t have F and the other leg is in F of a journey with transit always results in a F ticket for the whole journey, just that for the aircraft which doesn’t have F you sit in biz - for this situation there is no half J / half F price