First of all, welcome to the community!
I would encourage you to read through the
Welcome Guide if you haven't already. It gives a basic introduction to all this stuff.
I saw your question in the Rooting guide......and to answer your question: yes. You must already be rooted on a different software version (either ZVD or ZVH) or a custom ROM in order to root stock ZVJ. So in a sentence, you must revert to ZVD before being able to root ZVH.
To help with some of the terms:
Stock - this refers to any part of the phone (software, hardware, etc) that is in the same state as it was when the manufacturer and/or carrier (LG/Sprint) shipped the product. This also refers to any
original or
non-modified software/firmware that was distributed by the product's source or creator (like LG and Sprint) after the initial release (such as an over-the-air firmware update, like the ZVJ update)
Radio - the partition on the internal flash memory that controls the phone's Radio functions (such as calling, text messaging, and data)
Recovery - This is also a partition on the internal flash memory. It houses the recovery image, which is used to update firmware or reset the phone to its original factory settings. On custom recoveries, such as Xionia CWMA or Cannibal Open Touch, there are many other features, such as making nandroid backups. *note: only stock recoveries (official sprint recovery images) can flash Radio images*
i might as well tell you what a nandroid is as well...
Nandroid backup - a complete backup of the phone, including all relevant partitions EXCEPT radio. This can be used to save a "snapshot" of the phone in its entirety that can be "restored" or "flashed" onto the phone at a later date. These are useful when installing custom or aftermarket software, so that you can always revert your changes in case something goes wrong.
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To answer your other question: Yes, you will lose all the data on the phone when you follow the steps in the rooting guide and revert to ZVD. However, your data on the SD card (such as pictures, music, etc) will not be erased. Also, your contacts will be stored in Google Contacts, as long as you have your account on the phone synced with your Google account (to ensure this is the case, go to settings> accounts & sync and make sure your Contacts are synchronized).
When you set up your google account again after rooting, it'll even ask you if you want to restore your apps. They won't be restored with all the settings and data, but the apps themselves will be downloaded
Don't be afraid to ask other questions. I would encourage you to do lots of reading and question-asking so that you feel completely confident before you try to root your phone. That way you'll be less likely to make a mistake that could make things complicated.
Hope this helps!