#5 - Feb. 24, 2018, 11:32 a.m.
It's not possible to pre-purchase the expansion for one single World of Warcraft account more than once.
If you wish to pre-purchase the expansion again you'll need another WoW account upgraded to Legion.
As for the boost price, this was changed in April 2017. You can read more about it here:
https://eu.battle.net/forums/en/wow/topic/17615085726