This is the main problem with 3rd party TA - they own the ticket
You used the LH app to change the ticket
Stuff gets fouled up beyond belief
You got screwed
Everyone circle jerks and blames the other one for not informing/reissuing/plating etc
Even booking a LH ticket thru UA and then upgrading with PP thru UA means your ticket will be red and say you have to call customer service and if they re issue you lose your upgrade - and those 2 companies are DESIGNED to work together
You couldn't pay me to use a 3rd party portal (expedia, venture, chase, velocity etc to make a reservation in this day and age)
Your best bet at compensation is to have a glass of wine and laugh about it - because that is all I would count on. You will get an ulcer before you convince LH and/or Venture it was their respective faults