There have been no major changes to battle mechanics, or overworld mechanics yet. All pokemon are intended to be available, except legendaries. Not even postgame. There is a large variety of pokemon available, even from the start, and all are intended to be viable. Though, this will take time.
Pikachu -> Raichu Lvl 20 (Was thunder stone)
Sandshrew -> Sandslash Lvl 20 (was 22)
Vulpix -> Ninetails Lvl 25 (was fire stone)
Golbat -> Crobat Lvl 34 (Was happiness)
Gloom -> Vileplume Lvl 28 (Also creates Bellossom, have a spare party slot. This works like nincada evolving into ninjask and shedinja All others with text like this work the same.)
Diglett -> Dugtrio Lvl 25 (Was 26)
Growlithe -> Arcanine Lvl 25 (Was fire stone)
Poliwhirl -> Politoed Lvl 34 (Also creates Poliwrath)
Kadabra -> Alakazam Lvl 40 (was trading)
Machoke -> Machamp Lvl 35 (was trading)
Weepinbell -> Victreebel Lvl 34 (was leaf stone)
Tentacool -> Tentacruel Lvl 25 (Was 30)
Graveler -> Golem Lvl 30 (was trading)
Ponyta -> Rapidash Lvl 25 (was 40)
Slowpoke -> Slowbro Lvl 25 (Also creates slowking)
Shellder -> Cloyster Lvl 25 (was water stone)
Haunter -> Gengar Lvl 36 (was trading)
Onix -> Steelix Lvl 25 (was trading w/ metal coat)
Staryu -> Starmie Lvl 30 (was water stone)
Seadra -> Kingdra lvl 35 (was trade w/ dragon scale)
Scyther -> Scizor Lvl 36 (was trade w/ metal coat)
Pineco -> Forretress lvl 20 (was 31)
Slugma -> Magcargo Lvl 20 (was 38)
Larvitar -> Pupitar Lvl 20 (was 30)
Pupitar -> Tyranitar Lvl 40 (Was 55)
Lombre -> Ludicolo Lvl 25 (was water stone)
Nuzleaf -> Shiftry Lvl 25 (was leaf stone)
Aron -> Lairon Lvl 20 (was 33)
Lairon -> Aggron Lvl 40 (was 42)
Shelgon -> Salamence Lvl 40 (was 50)
Metang -> Metagross Lvl 40 (was 45)
Shuppet -> Banette Lvl 20 (was 38)