I used to work in Chengdu, but now I am back in Nanchong, and I want to transfer to social security.

You can open a new account in Chengdu and start paying social security. If you don't work in Chengdu in the future, you can transfer this social security back and merge it with the original application. Of course, you can also apply to transfer to Nanchong, but you have to pay social security in Chengdu, and you can only transfer to Nanchong after one month of successful payment, and apply for merger. However, Nanchong seems to have to handle it by itself. You can call Nanchong Social Security and ask if you can entrust others to handle it.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.