I have not heard confirmation but I am confident the menu corruption was at least one of them.
I can say the Greatest Hits version (2003) definitely had some updates. The Pixel Remasters drop on the eShop will launch at the same time worldwide - 8am PT / 11am ET / 4pm BST on 19th April / 1am AEST / 1am NZST on 20th April. The console versions of these previously PC and mobile-only versions of the classic games feature the ability to change between the original and the new reorchestrated soundtracks, change the default font to a pixel-based version, turn off random encounters, and adjust the amount of experience you can get in battle. Here's what the eShop says for the six games and the bundle: And remember, these are approximate for now - we won't know the actual size until the games are sitting on our Switch, ready to be played. All of the file sizes (below) have been taken from the eShop in Europe, North America, and Japan.
You might expect the games just get a tiny bit bigger with each sequel, but that's not necessarily the case.