This might help someone down the line, as I've recently gotten new sim cards and upgraded three of my lines from the 3G version of the grandfathered international data plan to its 4G/LTE equivalent.

Was a bit tedious but having gone through the process separately three times I can tell you how it has to be done.

The sim card part was the easiest, I got them from ATT store but they were unable to activate them as the plan didn't appear on their end - so advised to go through customer care (which I had expected). Previously I have had luck with some managers in ATT stores that had access to the customer care portal but apparently this access has been discontinued.

Updating the sim with regular customer care on the phone was reasonably straightforward other than security questions and them having to call the number to verify its you.

The next step was to update the IMEI of the new phone (even though my online account shows the phone I currently have which have been 4G devices for many years, ATT still had my device as a Blackberry Bold!), this is where it got complicated. Basically since my grandfathered plan was still a 3G version the system would not accept to add a 4G device. So the customer care person (this took many many calls and back and forth with international support to actually figure out) has to remove your grandfathered plan and add a random plan to be able to add the new device - have them notate the account (just in case) that you don't intend on changing plans this is just being done to upgrade to 4G.

Once this is done they have to transfer you (or you can call) international care tech support who will then verify you previously had the plan and will re-add without asking any questions or trying to trick you into another plan.

The main issue is you have to deal with regular customer care to update sim and device as international care can't do that, and basically these guys are clueless about these plan or how to add it and will say just about anything out of ignorance. If you're dealing with a total idiot that is refusing to follow your step by step just hang up and try again.

For reference I've still got the Blackberry standalone unlimited international data at $69.99. I've kept this as I don't want to add a voice plan to my account but the sim works just fine in android and other phones (have never tried to stick it in an iphone though - will try one of these days).

I know from friends that also have the plan (and confirmed by int'l tech support) that you can seamlessly swap out the iphone/android/blackberry add-on plans that are $64.99 without much hassle though again this can be done only by international support. Only blackberry apparently had a standalone plan (no voice plan component on account) though at $69.99.

Hope this will help some folks out there as I know many of you are worried about losing the plan. Also moving to a 4G plan other than increased speeds including LTE when roaming ups your data "cap" to 5GB from the 3GB previously. Went over 3.5GB this month roaming with no throttling SMS.

Also one of the phones on my account is a Blackberry Passport and getting a new sim card activated my BBM voice and video which previously I could never get to work.