Update -

I was able to book and use funds from my previously issued UA ticket.

Now for clarity, this was because I indeed had bought an AC fare. Agent specifically told me if I had bought an UA fare, funds could've only been used for UA fares. They said this is why i had no issue using the funds for my CM fare. (Posted about earlier)

They also told me BR had no discount Y fare they could book in, but did for discount J. (Basically as posted by another user, plating restrictions prevent UA from issuing ticket for BR discount Y fares)