@arfghSorry for the late reply but as you can see it is a long post.
The first thing you want to do is make some backups to USB.
1. Backup all your Saves to USB.
Settings>Application Saved Data Management>Saved Data in System Storage>Copy to USB Storage Device
2. Back up trophies to USB using PS-Phwoar!
PS-Phwoar!>Backup and Dumpers>Trophy Backup
When you run the payload look out for which username it says when it starts. It will say 'Trophy Data found for User:xxxxxx'
If it says your old PSN username you will need to change a folder name later when restoring your trophies. I'll cover that later on.
There is no progress indication when it is backing up your Trophy data so make sure wait for the Backup Complete Message!.
3. Make a backup using the official Backp/Restore in Settings>System>Backup and Restore to backup your current profile and saves.
Only choose your saves and system settings. Do not backup your games(unless you have a large enough USB drive)
It would also be a good idea to use SaveMounter to mount and backup your 'decrypted' saves just in case something were to go wrong.
This should not be needed but it is another way to guarantee that your saves can be recovered.
When you have backed everything up you can now initialize your PS4.
WARNING! If you have a LAN cable connected to your PS4, unplug it before you initialize!
The reason for this is because while you are setting up your PS4 after it has been wiped (even if you select skip for the internet connection) The PS4 connects to the internet without telling you and downloads the latest update file.
It doesnt install it but still, very sneaky Sony!
Now you need to set some settings.
While you are setting up your PS4 remember to go into Settings>System>Automatic Downloads and turn OFF everything in there.
Also go into Settings>Power Saving Settings>Set Features Available in Rest Mode and make sure 'Stay Connected to the Internet' is OFF and 'Keep Application Suspended' is ON.
Now change your profile Username to whatever you want it to be.
Now set up your internet connection so you can install PS-Phwoar! Remember to use AlAzif's DNS if you are not using ESP.
The next thing to do is to to block updates and then Activate your profile.
1. Open PS-Phwoar! and go to 'System Tools 2' and run Disable Updates.
2. Open PS-Phwoar! again and go to 'System Tools 1' and run SaveMounter + FTP (to load the ps4debug)
3. On your PC open PS4OfflineAccountActivatorGUI.exe and enter your PS4's IP and click 'Connect'
4. When connected, click 'Get Users'
5. When you see your Username change the accountID to your personal 16 digit Hex number that you got before.
6. Click '3. Set id & Activate' and when it says it has activated your profile, reboot your PS4.
When your PS4 has rebooted your profile will now be Activated. You can tell by the yellow star next to the trophy Icon on the top menu.
Now we will try and restore your Trophies.
As I said before, restoring your trophies might not work because of your old PSN username situation but there is a high chance that it will because you now have the same accountID.
1. Open PS-Phwoar! and run Hen. When Hen has loaded close and re-open PS-Phwoar!
2. Go to 'Backup and Dumpers' and run 'Trophy Restore'
3. If it says 'No valid backup found' This is because it used your old PSN username when making the backup.
To fix this open the USB on your PC and rename the folder inside the Trophy_Backup folder from your old username to the one you have now and try the Trophy restore payload again.
If it still says no valid backup found then you cannot restore your trophies.
If it does start to restore them, wait for it to finish then reboot your PS4.
To test if it worked go to the Trophy icon on the top menu of the PS4 home screen and after it says it cannot connect to PSN, select 'View Trophy Information saved on PS4'
If it worked then you can go on to Copying your saves back to your PS4 from USB using:Settings>Application Saved Data Management>Saved Data on USB device.