I've successfully upgraded from 4.2 to 4.3 Sense 5.5 yesterday afternoon. For those that want the update before going to kitkat or flashing Google play edition (losing Sense, ZOE, FM Radio, Blinkfeeds etc), here's how I did it:
1. Unlock bootloader.
http://www.htcdev.com/bootloader
2. Root phone
3. Use rumrunner (my hboot version is 1.54) to s-off phone
http://rumrunner.us/
4. Download 4.3 Sense 5.5 OTA update:
http://www.htc1guru.com/dld/ota_m7_...-24-401-8_release_338160gxmo5sd9337kadux-zip/
5. Save downloaded file to phone.
6. Boot to stock recovery.
7. When red triangle with "!", e.g /!\, hold power button + volume up to enter blue recovery screen. Select zip file and flash.
8. Profit.