[One (M7) Q&A, Help & Troubleshooting] LibertyMarine:
So, first of all.. in that case:

DON'T GO S ON! YOU MAY INSTANTLY AND INEVITABLY BRICK YOUR DEVICE

As far as I can estimate, you might still have some modded software on your phone that might result in a bricked device if you go s on. So it's the safest not go go back s on. Stay S-OFF!

What you've got to do is:

-Get locked

-Be unrooted

What they'll see because you are s off is a red Text somwhere located in the settings. But this won't be very bad for you simply because they almost can't void you any warranty at cause of s off. This is because HTC unintentionally ships s off'ed devices. And even if they'd say modded, you MUST fight! You'll fight them! Fight their decision until they give up, ask for the next guy at your shop until you got your warranty! I can give you some arguments that'll help you.. but this is one of the worst cases.. so don't talk too much about it right now.

How to get locked:

This here is an excellent thread that explanes you how to do that. But the prequisited will be offered by myself.. that's how I did it as a noob and I think that you'll understand these easy, simple explanations better than these complicated or incomplete instructions provided by the guy that made the thread I linked. (Somehow I think this isn't correct grammar.. sorry.. I'm from Switzerland) And these instructions will even be too easy for you.. but you'll understand them.

The proceed the steps in this thread, do this:

Copy the supersu.zip to your sdcard(Download: http://download.chainfire.eu/370/SuperSU/UPDATE-SuperSU-v1.80.zip)

Boot your phone in the bootloader

Select fastboot and enter into the fastboot menu

Plug in your HTC One in the PC

Wait until it's written FASTBOOT USB

Download a custom recovery(TWRP is the best) and store it in the same folder like fastboot.exe

Type in this command(CMD with admin permissions):

fastboot boot recoveryTWRP.img

If you want to, make a backup

When the custom recovery booted, you may flash the supersu zip

Boot your phone in the system... the recovery won't be written on the recovery partition, you only booted into it.

Proceed these steps starting at 2) reset your blabla http://xdaforums.com/showthread.php?t=2475914

You also need to have adb.. but I think you have that

Here is a quote:

Now boot into your system again

go to supersu app and open it

go to the settings and choose unroot.. or however it's called, there is only one option that is about unrooting

Do whatever the unroot wizzard ask you to do

if you are unrooted, reboot your device

Check in the bootloader that you are not tampered, locked, correct CID

now, if you are sure you are stock, go to the settings and run through all the OTAs

Now you are really completly stock except s off... the supersu installation is hidden, they won't find out that you rooted your device.

Tell me about every error.. and if you had succes, tell me about it. If you need help by finding arguments against voding warranty.. tell me. But let's wait. The chances are high that you get it done on warranty.

And before asking about the risks of doing this. There are! But they are sooo small.. the chances that you'll get killed by a flying fridge are higher! You'll be able to gain a unlocked device by using HTCdev with the unlock token you once recieved! And when doing this... simply don't do it if there's a storm outside.. someone is running around in your house who might falls over you cable and blabla... but if that happens.. first look that the person wasn't hurt by the accident and so on...

MAY THE FORCE BE WITH YOU