[Verizon HTC One (M8)] seakha:
Method and results of my process of converting Verizon M8 into T-Mobile M8

Almost four weeks ago I converted my Verizon M8 into a T-Mobile compatible version through the process I will explain below. I promised that I will post the result of my test. I did a three weeks of solid testing under different conditions and in different cities. After that, I converted our two other M8's in our family plan into T-Mobile version and all three are working.

Heads Up! This method might not work for you. I noticed some inconsistencies few days ago while testing the phone in Chicago. So, use it with your own risk. The conversion might not work for you.

Short result: I am happy, my phone works as a T-Mobile version with few glitches but with a lot benefits.

Pros:

4G LTE speed;

WiFi calling;

Updated features for camera, etc;

ViperOne features!

Every thing works including: GPS, MMS, and imporved battery performance on LTE.

Cons:

Some instability similar to any other unofficial ROMs you flash on your device;

Couple of incidents that my camera stopped working (fixed after turn off/on);

Once the GPS could not lock (fixed after turn off/on)

If you lose reception (e.g., in subway) the phone does not pick signals right away, sometimes you need to put it in flight mode and then it will pick signal again

I am living in Metro area and the LTE speed is a real game changer for me (I get up to 39 Mbps). I also have a few blind spots in my office, thus the WiFi calling is very useful for me too. Therefore, I am very happy with the conversion. However, it might not work as well in all cities. I tested the phone in two trips, in Puerto Rico and Chicago.

[*] In San Juan, Puerto Rico, the phone was working perfectly with good reception and data speed, but in the rural areas of Puerto Rico, I had not data and phone did not switch automatically to 2G. I had to do it manually, in network setting.

[*] In Chicago, I faced a very weired behavior of the phone. When I was not using the phone for a while it could lose reception and to reactivate it I need to put flight mode. It might be related to different bands used by T-Mobile LTE. I do not know and I have no knowledge about it. [MENTION=2787279]vprasad1[/MENTION] in this post discussed the issue. That's way I emphasize "use my method with your own risk, it might not work in your city." It works for me and I am happy!

Method to Convert Verizon HTC One M8 into Fully functional T-Mobile version:

1. Use Sunshine to S-OFF and Unlock bootloader the device.

2. Flash TWRP recovery to the device via fastboot (I used TWRP 2.8.3.0).

3. Root by SuperSu app (you can install the SUInstall provided by Sunshine and then install SuperSU app to update the binaries)

(Optional) 4. Run a Nandroid Backup (Not necessary but helps you to recover if something wrong happend). You can also use HTC Backup app to get a Backup of your apps, messages, call logs and your preferences.

5. Update firmware to the latest version (4.4.4) RUU to get firmware 3.28.605.4. This will wipe you data and get you to the stock ROM (I used method 2, copy zip file on a FAT 32 formatted external SD card). This is necessary, because ViperOneM8 3.2.1 requires the latest VZW firmware (4.4.4).

6. Install ViperOneM8 through TWRP recovery (I used ViperOneM8 3.2.1): wipe data to have a clean install; I selected T-Mobile version in the installation process.

(Optional) 7. I used HTC Backup to restore my data and layout and theme of my phone.

Footnote1: TWRP 2.8.3.0 was not working on 4.4.2 however it was installed, I could use TWRP manager for back up even. I used TWRP manager on 4.4.4 to reboot into recovery and then do the rest of installing process.

Footnote2: The first time I did these steps my phone did not have cellular connection. Nothing was wrong with the method. I found out the problem was caused by a stupid app (radio Switch) which apparently corrupted my NV partition. To fix the problem with the radio (no cellular signal) I went back to the base Verizon Stock rom (4.4.2, firmware 1.55.605.2) then used the stock recovery to do a data/factory reset. This restored the NV partition and fixed the signal problem. To do this I used RUU M8_VZW files (s-off only): [get you back to Verizon Stock rom (4.4.2, firmware 1.55.605.2)] (I used method 2).

Good Luck!