Before you get TOO excited, things like the Sea Monster Trio, Quests, and Post Game are NOT part of this but there are some new additions and fixes. Probably the biggest change is that the Elementalist Glitch is finally a thing of the past.
(For those who don't know, a bug in the Ability Elementalist applied a +50% boost to ALL Fire and Water-type moves used by ALL Pokemon. This is why you might have felt like Cali and Tiko were hitting harder than you were expecting, it's because they were. Or why Theo was so hard to beat when getting Orchynx instead of Raptorch or Eletux.)
Check the updates section of the Discord for more information on additions and bug fixes https://discord.com/channels/170029379458564097/229288136398733323
And be sure to run the patcher from the main menu to update to v1.2.5 to take advantage of the newfound changes.
(P.S. To anyone reading this looking to troll, there's a list of all the changes made to the game as part of the 1.2.5 update, so if you try to be cheeky and act like Duplicat's stats were changed to be all 69s or something, trust me, we'll know you're BSing us.)