With the age of Y-20, they will need to maintain two engines for quite a long time even if WS-18 will be replaced with WS-20 once those machine finish their life. Will be quite interesting to see the number of engines they can pump out, It's China first high bypass turbofan and for now used only on one plane.old engines will be replaced with WS-20 once those machine finish their life. remember J-10B/J-11 reengined with WS-10.
it will be logistics nightmare to maintain two engines on Y-20 juts like fighter jets. PLAAF rapidly replacing Russian engines.
WS-18 production will also be cutted a lot with only H-6 getting them in the future.