Question [Q] Imei problem

Hi guys
i have a Nokia Lumia 925 , when my os version was 8 , the imei shown by phone was exactly the same with the phone box
but since i've upgraded my phone to the latest os version 8.1 unfortunately phone imei has changed
now my question is do you know whats wrong with my phone ? and what should i do to repair that damn imei ?
can i downgrade my phone to wp 8 ? and how ?

Best Regards
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