If you look at how things are produced in Chinese factories now. Being flexible and using latest production tool to improve efficiency is a must. I don't see why aerospace sector should be any different.Retooling, supply chain management, retraining (production staff and maintenance staff) will all take large efforts with potential J-20 manufacturing changes. On top of all this you will have to recertify the aircraft which will take a substantial effort too.
I would think at this stage of the J-20 project it is probably not worth the effort to radically change the manufacturing methods used. Small changes, refinements etc. obviously will happen.
We only have 300+ J-20s in service. J-20A production might be 1000 over next few years. Why would you not do further investment for that?
And again, they built out new factory floor space at CAC. As such, they are not retooling. They are bringing in new tools.
The level of automation in China's industrial base is very high and growth. Don't underestimate China's ability to produce things.
The limitation to J-20 and J-35 procurement is more likely due to training pipeline and budget rather than production capacity at this point.