Right, the reasons why I involved UA:
- AC did not want to sell me new tickets.
- AZ did not want to sell me new tickets.

and two more reasons:
- I had a valid ticket with UA. That means that UA would have earned the revenue for my ticket if they had sent the ticket coupon to AZ.
- I wanted to have exactelly the same flight as on my tickets from AZ. I believe this makes it easier to proof that I was refused to check-in.


I did indeed try to solve the issue in a politely manner with all three airlines at EWR but none of the agents gave me a hand to solve the problem.

As for the sticker thing: I often travel with stickered tickets and I was never refused. The more it is to say that stickering is common policy in Europe. They may have re-issued the ticket if I had changed the whole routing but I did the change en route. Therefore, I strongly believe that:
- AZ in Milan did not make a mistake by stickering our ticket
- AC did wrong by not accepting our ticket (which was with a UA flight number)
- UA did wrong by not helping us. They are next to AC in EWR.