What if the unit is replaced by Shanghai social security?

If you are still in the same city after job-hopping, you need to ask the original unit to cancel your employment record and transfer the insurance to the temporary account of the city after signing the formal labor contract cancellation letter. The new unit will give you a new record, then transfer to insurance, declare the base and continue to pay.

The key is that you need to go through the resignation procedures and put your contract and employment records in your file. I come from Dalian.

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