Why not use a military plane to change it into a big one?

No, take Yun 9 as an example. Yun 9 is a military aircraft, and its maximum take-off weight is similar to that of C919. If it is to be changed, the problems are as follows:

1. Economic benefit

Yun 9 has four turboprop engines, while C919 is a twin-engine turbofan engine, needless to say, two engines and four engines save fuel. As a military aircraft, performance is the main thing, but for civil aircraft, safety and economy are the first. After a flight, the air ticket is not enough for gas. Isn't it a joke?

2. Environmental protection, noise, etc.

As a military aircraft, although environmental protection is considered, these are the last considerations. If Mexico bought Yun-9 and wanted to fly to the United States, the United States could shut Yun-9 out for various reasons, such as environmental protection, noise, safety and so on (that's how Concorde was killed in those days)

3. Unsafe Yun-9 even if it was changed to a civilian aircraft, some parameters of Yun-9 for military use and Yun-9 for civilian use were the same as those of a civilian aircraft. If the United States bought Yun-9, China would be under study.

4. There are many other things, such as different models, different fuel loads, different cabs and different Hangzhou Electric Power Company. If you want to modify them, you might as well start a new stove directly.