Today im going to show you how to change your summoner spells after you joined the game.
I dont know if this works with the new client.. so just start the old client
-First go in Champion Select pick everything you want
-after you picked your champion open the Summoner Spell window and leave it just like that
-Youre good to go! whenever you want to change your summoner spell just exit the game.. The Summoner Spell menu will still be open and you can select another Spell before you reconnect
This may not work with every game mode! I only tried in ARAM
keep in mind that you can only change it once per game. this is a bug but you may get banned so use it carefully
Have fun