Does the 15,000IP Summoner Name Change reroll your whole account?

No I just did it

No, there's a Summoner ID that Riot uses under the hood to keep track of who you are, regardless of what you change your name to.

Don't know why people downvoted you, the question might not be the smartest, but it's nothing anyone should feel offended about... anyway nope, you change your summoner name, not your account id (as you might know your summoner name can be different from the name you use to log-in) which is used to track and recognize your account, so by changing your summoner name you won't lose anything since it's not what riot (the game's database actually) uses to define who you are.