Niantic has announced some more details on GO Battle League, upcoming changes to the combat system, attacks that some Pokémon will be able to learn in Pokémon GO for the first time, and some familiar attacks certain Pokémon will be able to learn which will be coming to the game in future updates.

Pokémon GO - Community Note: GO Battle League preview, combat system update, legacy attacks!

Trainers, In this special Community Note, we’re excited to share some more details on GO Battle League, upcoming changes to the combat system, attacks that some Pokémon will be able to learn in Pokémon GO for the first time, and some familiar attacks certain Pokémon will be able to learn once again! Let’s GO!

Leagues, seasons, ranks, rules, and more!
Pokémon GO Battle League will be a triathlon-style contest!

• Battles will take place in all three leagues: Great League, Ultra League, and Master League. However, only one league will be available at a time, and they will all rotate throughout a season. This means that in order to be the best, you need to be able to win in all three formats!
• As you win battles throughout the season, your rank will increase, even as GO Battle League rotates between leagues. You will start at Rank 1 and work your way up to the top rank, Rank 10. Once you reach a new rank, you cannot drop below it for the rest of the season. Higher ranks mean better rewards!
• Once you reach Rank 10, measure against the best with your battle rating. Wins increase your battle rating, while losses will cause it to drop.
• You cannot use two or more of the same species of Pokémon on your team.

As we get closer to the release of GO Battle League in early 2020, you can look forward to more information on how GO Battle League works.

Updates to the combat system
There are some exciting updates coming to the combat system.

1. When two Pokémon use Charged Attacks at the same time, the Pokémon with the higher Attack stat will go first. If two Pokémon have the same Attack stat, a random Pokémon will go first. We want to ensure Charged Attack ties are broken fairly. Restrictions in Great and Ultra League currently put a focus on trying to get low-Attack Pokémon to maximize CP. While maximizing Defense and HP is a great strategy against most opponents, in close matchups, the Attack stat can now become a deciding factor. We hope this will help deepen team building!
2. Switching Pokémon now clears any changes to its Attack and Defense stats. As more attacks are added to the game that affect Attack and Defense, we want to ensure these effects have clear counterplay opportunities. Debuffs should encourage players to switch Pokémon and buffs should make switching less appealing. There’s more to consider now when to use these attacks and when to switch Pokémon.

Updates to existing attacks in Pokémon GO
There have been updates to the following existing attacks in Pokémon GO.

• Night Slash - Its energy requirement has been reduced in Trainer Battles, and it now has a chance to greatly increase the user’s Attack stat.
• Icy Wind - This move now decreases your opponent’s Attack stat when used.
• Hydro Cannon - Hydro Cannon now deals less damage in Trainer Battles. Community Day exclusive attacks are intended to be powerful, but Hydro Cannon was too strong.
• Frustration and Return - They now take longer to charge in Trainer Battles and deal more damage. Since all Shadow Pokémon have access to Frustration and Return (through purifying), we wanted to make sure these attacks do not become too all-purpose.
• Mud-Slap - Damage for Mud-Slap has been increased in Gyms, raids, and Trainer Battles.
• Sand Tomb - When used in Trainer Battles, Sand Tomb now charges quickly, deals less damage, and decreases your opponent’s Defense by one stage.
• Forretress, Torterra, Garchomp, and Gliscor can now learn Sand Tomb.
• Earthquake - Its damage has been increased in Gyms and raids.
• Overheat, Draco Meteor, and Psycho Boost - These attacks now charge up quicker in Trainer Battles, but they sharply decrease your own Pokémon’s Attack after use. Some damage values have been adjusted down to maintain balance.
• Close Combat - This attack now charges up quicker in Trainer Battles, but it sharply decreases your own Pokémon’s Defense after use.

Attacks new to Pokémon GO
Certain Pokémon will soon be able to learn attacks that hadn’t yet been seen in Pokémon GO!

• Octazooka - It’s a slower Charged Attack that may sharply reduce your opponent’s Attack.
• Octazooka can be learned by Octillery and Kingdra.
• Mirror Shot - It’s a quick Charged Attack that may also reduce your opponent’s Attack.
• Mirror Shot can be learned by Forretress, Magnezone, Ferrothorn, and Klinklang.
• Fell Stinger - It’s a quick Charged Attack that also increases your Attack after use.
• Fell Stinger can be learned by Beedrill, Qwilfish, Vespiquen, and Drapion.
• Aura Sphere - It’s a powerful Fighting-type attack.
• Aura Sphere can be learned by Lucario.
• Superpower - It’s a powerful Fighting-type attack that charges up quickly but reduces the user’s Attack and Defense after being used.
• Superpower can be learned by Pinsir, Snorlax, Hariyama, Rhyperior, and Melmetal.

Pokémon that can learn more attacks
The following Pokémon will be able to learn attacks they couldn’t before.

• Cloyster: Icy Wind - Cloyster is a defensive Pokémon, and gaining access to the newly improved Icy Wind will help it fill that role even better.
• Feraligatr: Ice Fang - By focusing on its bite, Feraligatr can stand out a bit more from the other Water-type first partner Pokémon!
• Porygon2 and Porygon-Z: Lock-On - Lock-On generates energy very quickly at the cost of dealing very little damage. Porygon2 and Porygon-Z both rely on slow but strong Charged Attacks. Lock-On should help improve their chances of using these attacks.
• Celebi: Seed Bomb - Since Celebi is a Mythical Pokémon that has the Grass type, we wanted to make sure Celebi has access to a Grass-type attack.
• Shiftry: Snarl - Access to both Razor Leaf and Snarl should let Shiftry pressure popular Ultra League Pokémon like Swampert and Giratina.
• Kyogre: Surf - Currently Kyogre’s Charged Attacks are very slow. Surf will give it a faster charging option that matches its type.
• Lickilicky: Shadow Ball - Lickilicky is great against Ghost types already, and Shadow Ball will help it even more.
• Tangrowth: Power Whip - Tangrowth should be able to use its Grass-type Charged Attacks more frequently with this powerful move. Power Whip is also a great fit for a vine-covered Pokémon.
• Heatran: Flamethrower - Flamethrower should allow Heatran to more effectively combat Pokémon that are weak to its types, like Venusaur and Registeel.
• Stoutland: Ice Fang - Ice Fang will help Stoutland battle differently from other Normal-type Pokémon with Lick.

Pokémon that can learn old attacks once again
As part of the group of Pokémon to debut with Pokémon GO, some Pokémon originally discovered in Kanto have had their attacks change a lot over time! We’ll be adding back some of the stronger attacks a few of these Pokémon used to learn back in 2016 to make sure these attacks are accessible to players who have joined since then. We’ll look at bringing back more of these attacks over time.

• Arcanine: Flamethrower
• Poliwrath: Mud Shot
• Machamp: Cross Chop
• Golem: Ancient Power
• Gengar: Shadow Claw
• Exeggutor: Confusion
• Pinsir: Fury Cutter
• Gyarados: Dragon Breath, Twister
• Snorlax: Body Slam
• Dragonite: Dragon Breath, Dragon Claw

We’re excited to see the teams you build with these changes, Trainers. Stay tuned, as we’ll be sharing more updates like these in the future. Good luck on the battlefield!

Source: https://pokemongolive.com/en/post/combatrebalance-dec2019/