red: squirtle → nidoran
yellow: pikachu → nidoran
gold: toto
crystal: toto → raikou
sapphire: mudkip → kyogre
emerald: mudkip → rayquaza
frlg: squirtle
(colo: espeon + umbreon → espeon@quilava
XD: eevee + teddi)
dp: chimchar
plat: chimchar
bw: snivy → pup
bw2: tepig → drilbur
XY: chespin → farfetch'd → hawlucha → lucario
ORAS: mudkip → lati*s → boxart legendary
SM: popplio → lunala
USUM: popplio → hawlucha
LGPE: starter → starmie
Sw/Sh: sobble (→ eternatus)*

17(+2) games

12(+1) main swap, 5 of them being starter → legendary
5(+1) starter only

*assuming swsh as starter only because eternatus is used for one fight only