From Airlineroute:

Cathay Pacific in the last few days gradually outlined planned operation with its re-configured Boeing 777-300 and -300ER, featuring 3-4-3 layout in Economy Class (instead of 3-3-3). The re-configured aircraft is gradually loaded in the reservation system as well as the OAG, starting June 2018.

Current schedule is updated until 28OCT18, and service entry date is likely to move forward.

777-300 (J42Y396)
01JUN18 – 30JUN18 (Selected flights only for following routes)
Hong Kong – Bangkok
Hong Kong – Manila
Hong Kong – Osaka Kansai
Hong Kong – Singapore
Hong Kong – Taipei Taoyuan
Hong Kong – Tokyo Narita

01JUL18 – 27OCT18
Hong Kong – Osaka Kansai CX506/507

777-300ER J40W32Y296
eff 19JUN18 Hong Kong – London Heathrow
eff 01JUL18 Hong Kong – Rome
eff 01OCT18 Hong Kong – Johannesburg

777-300ER F6J53W34Y201
eff 01AUG18 Hong Kong – Tokyo Haneda CX548/549