Exclamation [Q] Help me guys! Service Centre saying new problem may occur

I gave my Xperia Z(C6602) 13W16 AAE for servicing to the Sony service centre on Feb 3 14. It lost sim detection and the IMEI gone blank with Baseband gone. It was working fine till I updated from 4.1.2 to 4.2.2 . I was on 10.1.A.1.434 firmware. It was for updating to 10.3.A.0.423. It was working for a day after updated to 4.2 . Then while on call, the call got disconnected and it said "Insert Sim card". Then that was it. Wifi was also not working. I got a new replacement mobile on Feb 22 14. It is 14W04 AAL. It's been working fine up until now; 4.3 version 10.4.1.B.0.101 firmware. Yesterday, a person from the Sony Service center called me stating that he got an IMEI error for my mobile on his e-mail. He asked me to come with the mobile and that he has to do something to it to prevent the future IMEI error on my mobile. He also said I'd get my mobile back within 2 hours. I don't think I should go. I think they're trying to cheat. Like taking back the phone and do something to firmware, bootloader, anything... which would make problem later on OR giving back a refurbished mobile saying "Sorry we couldn't do anything to it". kindly help me out guys.