Transmission slipping after major overhaul

Before you do anything else – have you checked the transmission fluid level? If it’s low, it’s possible that’s the only reason you’re having problems. If all they replaced is the torque converter and valve body, then it’s unlikely they did directly caused the problem (as long as the correct parts were used). But transmissions can be temperamental. If you had some clutch packs that were almost worn out but barely hanging on, and now suddenly the transmission is full of fresh clean fluid instead of older fluid that had lost some of its lubrication, this could be why those clutches are now suddenly starting to slip (especially when it’s cold – once they warm up the tolerances will probably increase enough to work again consistently).

That said, assuming the slide continues to worsen, you have a few options.
1. Fight with the dealer and SOA to repair free streaming. I doubt they’ll go for it, but they might take pity on you and offer a discount for a replacement, or extra credit for trading in the car. Slipping clutch packs or broken gears require almost total disassembly of the transmission and this requires a lot of time, special tools and knowledge that dealers rarely have. That’s why they’re only offering you a new stream right now – a direct switch is easy.
2. Take the car to a good independent transmission shop and have them replace it with a good used transmission or rebuild your transmission. An exchange will be much cheaper than a rebuild.
3. Replace a used transmission yourself from a junkyard. 5EAT transmissions are generally pretty solid, if you get a working transmission from a car that was bottomed or something, you’d probably have luck with it.
4. Trade the car.