The need of that kind of aircraft is so high that they will still build more when the d30 ones will be scrapped... I don't think they will be retrofited.Will the Y20 airframe outlast the engines? If so, will retrofit with new WS20 be easy?
Probably they still have stocks of Russian engines for the existing Y20. It makes sense that new build Y20 will use WS20 engines.The need of that kind of aircraft is so high that they will still build more when the d30 ones will be scrapped... I don't think they will be retrofited.