Maybe its been like this awhile, but we can use our level 100 boosts on any account attached to our bnet account. I seem to remember having to use them on the account I upgraded, but now I have 4 level 100 boosts I can use from any account.

World of Warcraft®: Legion includes a Character Boost that levels any character on your Battle.net account to level 100.
The profession is new as well, the character I boosted when I prepurchased the collector's edition has no professions.
If the character is at least level 60 before the boost, their primary professions and First Aid will be leveled up to rank 700. If they don't have primary professions, they are given class-appropriate professions according to the table on the right.
https://us.battle.net/support/en/article/7623