Yang Chengfu (1883 -1936),
Cheng Manching (1901 or 02 -1975)
Cheng Manching studied Tai Chi with Yang Chengfu from 1928 to 1935. Cheng allegedly ghost-wrote Yang Chengfus book "Tai chi Chuan Tiyung chuan-shu, published in 1934.
Yang Changeful died in 1936. I doubt they were having discussions about changes to Yang style then. Chengfu was discussing additions to the Yang family Taiji, meaning an additional form but not changes to existing forms. I have also read nothing that says Chengfu approved Chengs changes.
By 1945 or 46 I believe Cheng had pretty much developed his style, he began teaching in Taiwan in 49 or 50.
I find it highly unlikely Chengfu approved such changes since they are major bigtime fundamental changes to Yang style, which I am pretty sure Chengfu would not approve. The Yang family until recently never even acknowledged it as tai chi and when they finally did they said it is not Yang style. This also says not an approved change. Unless it is discussed and approved by a Yang family member it is not Yang style.
Tung Ying Chieh (1898-1961) taught Yang style and he designed forms after Chengfu's death that he called Tung style and it appears the Yang family has never had a problem with this. I also do not beleive the Yang family would have had a problem with Cheng Manching style if he had called it that and not called it Yang style.