Nope, you're stuck once the upgrade happens.
I too hated my S5 after the upgrade to 5.0.0, I'm still not fond of it. Well, actually, everything was fine at first. Battery life seemed to be about the same, performance felt smoother, and the added notifications on the lock screen were a bonus. That slowly changed to where my S5 would maybe last for 6 hours on a charge if I did absolutely nothing, it would access all sorts of stuff while in my pocket. It once opened up Facebook, sent a friend request out, has Facebook messenger open, had my contacts open, dialed someone at 4:30 AM their time, adjusted my Google Music app settings to allow streaming over a cellular network, started streaming songs, had Google Messenger up, and was trying to access a website in Chrome all at the same time. I was outside one morning at 6:30 AM cleaning the snow off of my car. Half an hour later, when I was done, my phone was burning up in my pocket. It had all of that crap going at once and the battery had gone from 100% to 20% in just 30 minutes.
I broke down completed a factory restore. That has fixed a lot of the issues. Performance has been fine the last two weeks, battery life a lot better (still not as good under Android 4.4.4), and it stopped doing random stuff while in my pocket. The only problems, for me, that weren't fixed with a factory restore were app crashes (I am constantly getting an error message saying that Contacts has stopped working) and the heart rate monitor still comes on if I have my finger over it despite not running an app that uses it. Battery life could still be better. Before, with 4.4.4, I could play music for 4-6 hours a day, text, and barely get down to 73% by the time I left my office. Now it's down to 65% by the time I leave regardless of what I do. That's still better than before when it would get down to 10% when I was done, that's if it even lasted that long.
Samsung's approach to Android 5.0 has me hesitant to buy the Note 4/Edge as I don't want the same things to happen again.