iPhone Possible to upgrade iOS on 3GS that was jailbroken, unlocked last year?

You can legally unlock it thru several unlocking services. This unlocking will be permanent. However, keeping the jailbreak will be a great deal of trouble, since I don't think the current latest iPhone OS can be jailbroken and remain untethered.

Everything you need to know about IMEI unlocking is here

AT&T is the cheapest to get unlocked.

https://forums.macrumors.com/threads/1398957/

Pay the $2 and get an imei unlock for it.
It's permanent and easy.

Little-Acorn said:

I just called AT&T directly, 'fessed up, and asked them if it's possible to unlock this phone legally somehow, so I can upgrade the iOS and still be able to use T-Mobile and use the GPS.

They seemed to say that I had to be a previous A&T customer. I got this phone on eBay and don't know its ancestry. Hmmm.

Do any of these "several unlocking services" you mentioned, not require the phone's pedigree?

Or maybe I just ran into an AT&T clerk who hadn't had her morning coffee, and wasn't giving me the whole story?

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You can try swiftunlocks if AT&T won't unlock the device for you.

By all means, they should. Try calling again and explain how you bought the 3GS from someone who you don't have contact with anymore and would like it unlocked. They'll ask for the IMEI if they will unlock it for you. Maybe you just talked to a bad representative

https://www.swiftunlocks.com/brands/apple

darricksailo said:

You can try swiftunlocks if AT&T won't unlock the device for you.

By all means, they should. Try calling again and explain how you bought the 3GS from someone who you don't have contact with anymore and would like it unlocked. They'll ask for the IMEI if they will unlock it for you. Maybe you just talked to a bad representative

https://www.swiftunlocks.com/brands/apple

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They don't have to unlock it for you if you're not a customer and you don't fulfill the following requirements.
https://www.att.com/deviceunlock/client/en_US/

General Eligibility Requirements for Unlocking All AT&T Mobile Devices

AT&T will unlock an AT&T Mobile Device under the following circumstances:
• The person requesting the unlock must be one of the following:◦ a current AT&T customer
◦ a former AT&T customer who can provide the phone number or account number for the account

• The AT&T Mobile Device was designed for use on AT&T's network.
• The AT&T Mobile Device has not been reported lost or stolen.

Account is in good standing (i.e., it has no past due amount or unpaid balance owed AT&T), and all contract obligations, including any Service Commitment, associated with the device to be unlocked have been fully satisfied. The Service Commitment associated with a given device may be satisfied in one of the following ways:◦ Fulfillment of the Service Commitment by expiration of any contractual term
◦ Upgrading to a new device under AT&T standard or early upgrade policies
◦ Payment of any applicable Early Termination Fee



• Postpaid Smartphone Customers: Account has been active for at least ninety days and is in good standing (i.e., it has no past due amount or unpaid balance owed AT&T.


• Prepaid / Go Phones: Account has been active for at least 6 months.

So best bet would be to pay swiftunlocks the $2 and get it out of the way