Mega Gardevoir to drop to B+: yes. See the two nomination posts.
Klefki to drop to B-: yes. It's too predictable which makes handling it not too difficult
Landorus-I to rise to B: yes. It's a very strong attacker and it bypasses Intimidate/WoW unlike every other Ground-type. You have to get around forgoing Lando-T though but otherwise it's fine
Zapdos to rise to B+: yes. Tbf it kinda fits between all the B mons and the B+ mons. It's a bulky Tailwind that has good SpA too and good typing too. If Suicune's in A rank, Zapdos deserves this.
Slowbro to drop to B: yes. I can use Cress as a bulky TR user, or Jellicent if I don't want to use the Mega slot, and these days, the Mega Slot has a high opportunity cost. If only MegaBro's ability was better
Raikou to rise to B-: yes. Helpful if you want a fast Electric-type with good bulk, better stat spread than Thundurus-I. Kinda suffers from realising when it's more useful on the team than another Pokemon, but that's why it's B- and not higher.
Mega Altaria to rise to B-: no. It really needs to get a DD boost to get going. 110 offence is kinda a let down for a Mega, even with Pixilate, and 75/110/105 isn't a standout for a Mega either. Not that its stats are bad, but I would really like either more power or more bulk to make good use of the Mega slot when it faces competition even with non-Mega Pokemon like Azumarill as a Fairy physical attacker.