Its fine to update to d-rex over rex as long as it says your Lv2 & target type are both DEX in rebug toolbox.
No spoofer need to sign into PSN on 4.75.1 D-Rex
To get you original cid back that came with the console use your dumped NORBIN/NANDBIN that's on the usb stick you used to convert to DEX.
If you deleted it just dump the flash again to usb stick in Rebug toolbox.
Rename it to have .bin extension on the end so FLASH-NOR-FWXX.X.NORBIN.bin
Then simply drag your nor or nand file into PS3 console id to get your IDPS & PSID.
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Use ccapi to set the cid to auto load at every boot
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