More than likely a new phone will come with LP already installed. However, if it does happen to come with KK, then visit this
XDA THREAD to learn how to block OTA and Knox.
If the phone comes with LP and the build version ends with BOAF, then the phone may still be able to be rolled back to KK. Visit this
XDA THREAD for reverting back to KK.
If the build version ends with BOF1, your phone is stuck on LP as VZW has incremented the bootloader. You cannot revert a BOF1 phone.
If the phone does come with KK and you want to try LP, there is a thread for a "
Safe Upgrade" which allows you to revert to KK again after trying LP.
Whatever the case, it's highly recommended you read through the linked threads so you get a full understanding of what you need to do. Flashing always has risks, but the linked threads have worked for me with no problems. YMMV!