Need to upgrade from Android ver. 4.4.4 to Android ver. 5.0 with unlocked bootloader
Actually it's possible to relock the bootloader, using Flashtool. The process can be found on this XDA Forum post.


Download Flashtool from www.flashtool.net
Start Flashtool and click on BLU button
You will be asked to connect device in flash mode
Once you connect the device you will get a prompt to "Relock"
Click on it and when prompted disconnect the device