Think twice before you make the move to UAL
After flying UAL almost exclusively for two decades, I moved to Delta about 18 months ago.
My experience is, on the whole, Delta is much better than UAL.
The free domestic upgrades on Delta are fantastic, the flight attendants are generally more helpful and friendly, the food is better, and if there is a problem, Delta is faster and fairer in resolving it.
Recently I used all my remaining UAL miles to fly business class JFK to Singapore. After several flights and many hours in business class, I can tell you that UAL's service is wildly uneven. You happened to hit a good one, but sadly, this is not the norm in my experience.
For me UAL's only advantage is that Star Alliance is stronger than SkyTeam. Other than that, Delta wins hands down on almost every front.
I'd try to resolve your problem with Delta, and if that works out, I'd stay put. Or try a couple more flights on UAL before you make the big more. I was hugely loyal to UAL and have many fond memories of my flights with them, but today, Delta is a better choice in my opinion.