As has already been discussed extensively, the parts are accurate down to the micrometer, hence interchangable.Ok, clearly you have misunderstanding about concurrent engineering. It is not making the change during the production stage based on customer's feed back.
The main concept of concurrent engineering is not only making every sub-system contractor to start his part of R&D simultaneously, but bring all other department (i.e procurement, maintenance, factories, airforce, etc) to participate the design work from very beginning. So, these departments can start to design their part of procedure along with designing team, as the result, they can save their time. On the other hand, the design team can receive their feedback immediately and incorporate these into their blueprint before the "final product". So, it can reduce the time-consuming and expensive design change in IOC.
In other words, one of major purpose of concurrent engineering is to minimize the modification after IOC.
If someone blames the modification after IOC for the delay of the project, the concurrent engineering approach didn't work as it is supposed to be in this project.
You are the same guy who was denying Uyghur enslavement in support of the Chinese government in the China Economy thread.
Mods, Chinese troll alert. Repeating the same misinformation/propaganda over and over again constitutes as trolling.