Hi, I got a used miner, it seemed to have worked fine for some time, but suddenly yesterday it changed mining address and pool, I tried flashing it with SD card but nothing changes.
Are you sure the Sdcard flash was done right? did you try to flash another working miner to confirm that both the files and the Sdcard function well? I remember someone mentioned that some viruses can corrupt the boot loader which is used to flash the firmware from the Sdcard to the DRAM, and so if that is the case, the control board is bricked for good.
To safeguard the other miners in the network, if I change the admin password of the other miners will they be safe?
No, it's not totally safe, you need to isolate this miner, use a different router / Vlan until you confirm the virus is gone completely.