Hi RoyalX,
In case it's useful for other folk that are in the same position I was, here is a blow by blow account of what I did to get my unlocked/unbranded Samsung Ativ S with Giffgaff UK SIM upgraded from Windows Phone 8 to Windows Phone 8.1:
1. Backed up my Samsung Ativ S:
a. Under "Settings", "Backup", "App list and settings", turned on "Backup", and initiated "back up now" to the cloud.
b. Under "Settings", "Backup", "messaging", turned on "Text message backup" and "Chat backup" to the cloud
c. Took lots of screenshots of all settings for the phone and various apps, just for my future reference
d. With the phone connected to a PC via USB cable, took copies of the entire contents of both "Phone" and "SD Card" storage areas
2. Used the Samsung Diagnostic tool to change the Country Specific Code (CSC) from "XEF" (France) to "BTU" (United Kingdom, where I actually reside):
a. Launched the Diagnostic tool by dialing ##634# from the phone pad - a new black window opened with a numpad
b. Entered *#321647335 and select "BTU". Pressed "OK" to reset the phone (back to factory condition!). Changing the CSC in this way results in phone reset and the loss of all settings and data in the "Phone" storage area. I realised later that data in the "SD Card" area is in fact preserved.
c. After phone rebooted, created a new App password on Microsoft website in order to login on the phone with my Microsoft account (since I use 2 phase authentication on my Microsoft account)
d. Restored the Phone backup from the cloud that was created in step 1 a. above
i. Had to provide passwords for Hotmail and various other restored email accounts. Since I use IMAP for those other accounts, I didn't lose any email.
ii. All previously installed Apps were automatically all re-installed from Microsoft store.
e. Some Apps such as WhatApp and Skype automatically retrieved previous messaging from their servers. Others lost all their configuration data, and so I used the screenshots saved in Step 1 c, to reconfigure them. Uninstalled a few Samsung Apps which the reset had re-instated. Re-downloaded maps.
f. Reconfigured the Microsoft Authenticator App used for 2 phase authentication, by re-pairing it with my Microsoft Account.
g. Went through all other apps briefly, so see whether they launched. All seemed OK.
h. Went through all settings, adjusting them to align with saved screenshots from Step 1 c. This included setting up a Phone lock screen PIN number again
i. Under "Settings", "Backup", "App list and settings", initiated another "back up now" to the cloud.
j. So at this point I was now running the same original Windows Phone 8.0 (OS version 8.0.10517.150, Firmware version 2424.13.10.1), but with CSC now successfully changed to "BTU" (as evidenced by entering *#1234# into the Diagnostic App), all my Apps re-installed and reconfigured again, and backed up.
k. Note: following this reset, a huge amount of space was recovered in the "Phone" storage (some 5 Gb)!
3. While I was performing step 2 above, my phone in parallel automatically discovered and downloaded a later version of Windows 8.0 (OS version 8.0.10532.166), which seemed specifically designed to prepare for subsequent upgrade to Windows 8.1
a. Selected "Upgrade" to allow this upgrade to occur
b. Phone rebooted to showing two gears turning, and then "ALMOST DONE Migrating your data (Step x of 6)" and finally "Update completed!" message
c. Phone was now running OS version 8.0.10532.166. Firmware Revision number was still 2424.13.10.1.
d. Under "Settings", "Backup", "App list and settings", initiated yet another "back up now" to the cloud.
e. Under "Settings" "phone update", unchecked "Automatically download updates if by data settings allow it" (to give me a bit more control over when download occurs)
4. Shortly after performing Step 3 e, my phone in automatically discovered the update for Windows 8.1 (OS version 8.10.14157.200) and displayed "An update is ready to download".
a. I allowed the "Download", after which phone automatically started preparing for upgrade.
b. Phone stayed on Update Status: "Preparing to install x%" for quite a long time (almost 10 mins)
c. Phone finally displayed "Your update is ready to be installed" (Postpone or Install)
d. I selected "Install" to install Windows Phone 8.1 (OS version 8.10.14157.200), and "SAMSUNG Update for Windows Phone"
e. Was presented with "Microsoft Service Agreement" dated March 2014, which I accepted by clicking on "Install"
f. Phone said "Restarting?", "goodbye", and rebooted automatically to showing two gears turning
g. Phone rebooted again, and I entered my PIN number when prompted
h. Phone displayed "ALMOST DONE?and it will be worth the wait, Migrating your data (Step x of 21)"
i. Phone displayed "Update successful", "Continue", which I selected
j. Phone displayed "New Settings" for "Wifi Sense" and "Additional Settings" to which I made some adjustments to the default settings
k. Phone now seemed to have all my Apps still intact! All App configuration data was preserved (Apps were apparently not re-installed from the Microsoft Store as in Step 2 d ii above, but must have been handled in a smarter way in Steps 4 b and 4 h above)
l. Phone is now running Windows Phone 8.1 (OS version 8.10.14157.200), Firmware revision number has changed to 2424.14.9.3, Radio software version has changed to 2424.14.9.3 !! Success!!
m. Reconfigured my Start Screen using Windows Phone 8.1 Start Screen folders to group related Apps (a big improvement over Windows Phone 8.0)
n. Phone "Backup" option has been enhanced to backup more things (e.g. browser favourites, Start screen layout, etc. and App content and settings to OneDrive, so did a "back up now" operation, to back all those things up.
So now my Samsung Ativ S has a new lease of life!! I have been very happy with this phone which I bought back in 2013 on Windows Phone 8.0, and am impressed by the fact that Samsung has continued to support it so as to allow it to be successfully upgraded to Windows Phone 8.1 with Cortana. If it subsequently is able to receive a further update to Windows Phone 10 when that becomes officially available, I will be doubly impressed!