1) Buy 2nd/used set should ask for receipt of purchase original or copy in case for future matters.

2) Should be ask phone is 1st owner or family or 2nd or 3rd parties n so on.

3) Should be ask this phone sign by contract at what Telco Company.

4) Should get all the details from 1st owner, if rejected suggestions better don't buy n looks for others.

5) If this issue related to 2nd owner or others parties when purchase better write or whatsapp or sms for the dealing agreement.

6) Nowadays so many signing contract at my telco so that can enjoy new phone, after that billing came then sell off the phone n money gone for other's matters as well.

Good Day Bro/sis.

Cheers

Normally change new mother board should be different imei even though refurbished/reconditioned set also have different imei.
I think sme didn't change mo or maybe change other's parts.

Should have service sheet from sme.

Cheers.