Question about expansion boost.

Sorry if this was asked before, I did a basic search but didn't wanna dig too deep because I was afraid of spoilers.

I started playing the game right before the Hutt Cartel Expansion came out then I took a break, I've been following Knights of the Fallen Empire for a while and it caught my eye so I went ahead and bought the Outlander starter pack.

My question is, how does the boost affect my experience? I'm familiar with the game mechanics as I played for about 3 months in 2013 and I've been playing WoW for 7 years, so this game feels very familiar in terms of gameplay and mechanics.

Do I get given a bunch of random choices for my legacy story, can I go back and do it anytime or does the boosted character have no legacy story at all?

Thanks! :)