The official Pokémon website and Niantic have announced that previously Mega Evolved Pokémon now require less Mega Energy for subsequent Mega Evolutions in Pokémon GO and that the amount of Mega Energy that Trainers receive for successfully completing a Mega Raid has been increased.

Mega Changes to Mega Evolution in Pokémon GO

Learn about changes to Mega Energy requirements in Pokémon GO, and what Mega Evolution changes will soon take effect.

Since Pokémon GO launched the Mega Evolution feature in late August, Trainers have had the opportunity to collect Mega Energy by battling in Mega Raids. The ultimate goal, of course, is to give yourself an edge in Raids by Mega Evolving select Pokémon. However, certain aspects of Mega Evolution required some fine-tuning, so we've implemented a few changes to the process.

To begin with, Pokémon that have previously Mega Evolved now require less Mega Energy for subsequent Mega Evolutions. We've also increased the amount of Mega Energy that Trainers receive for successfully completing a Mega Raid. These changes should make it easier to continue Mega Evolving your Pokémon.

We'll be implementing additional changes soon, including new ways to earn Mega Energy outside of Raids. Soon, you'll be able to earn Mega Energy by walking with your buddy and by performing research tasks. Trainers will also receive bonus Candy when catching a Pokémon that is the same type as your Mega-Evolved Pokémon. In the meantime, keep earning that Mega Energy, keep Mega Evolving, and keep on being the best Pokémon Trainer you can be.

Source: https://www.pokemon.com/us/pokemon-news/mega-changes-to-mega-evolution-in-pokemon-go/