Listed here are all the differences in the 2018 rerelease of the Generation 1&2 Tamagotchi when compared to the original vintage release.
This thread will be updated as more are discovered.
Shell design changes
First and most importantly, they replaced the two 357/LR44 batteries with a longer-lasting, less-likely-to-leak CR2032.
Reset button has been moved from the left side of the back of the shell to the right side. It's also now a plastic button rather than rubber.
Speaker design is connected via springs instead of wires. This might be the cause of the more muffled sound.
A, B, and C buttons feel much more responsive.
The gotchi runs at a faster speed.
Game design changes
Animations seem to be more consistent compared to the original Tamagotchi units. For example, when eating or expressing happiness, it seems to be a consistent 6 frames of animation instead of sometimes cutting off in the middle of a frame.Transitions and animations all feel more fluid and smooth.
The minigame on Gen 2 has changed. The left number is now randomized instead of being whatever number was shown on the right in the round prior.
Calls for attention seem randomized. I could be wrong on this one, but with the original Tamagotchi, they would call for attention (allowing you to raise their discipline meter) in a consistent pattern: Two hours after they evolved from baby to toddler, four hours after that, six after that, and eight after that.
Sound changes
The intro jingle to the minigame has been sped up, leading to a small gap of silence between when it ends and when the game starts.
Care Mistakes
The 2018 re-releases are far more likely to die due to care mistakes. This seems to be connected to how many snacks they are given, so avoid to many snacks at their baby, toddler and child stages. Play with them to fill happy hearts instead.