I imagine you were rebooked from CX390 to KA900 https://www.flyertalk.com/forum/cath...and-china.html

The CX 77K has 40 Business Seats and 32 Premium Economy seats
The KA 333 has 42 Business Seats

So very likely the Premium Economy pax will be reaccomodated in the back instead of the front.
Doesn't always work that way - if the plane was switched from a 77K/A into a 77P it's very likely PE pax will go up front.