Yes, if you unlock a branch you get to keep it. You don't even need to unlock a whole branch before you invest in a new branch (though usually I unlock the whole tree as soon as I can to get the benefit from completing the branch).
The only thing with social policies you have to be aware of is that some branches can't be active simultaneously. In other words, you can't have freedom and be a nazi at the same time. Another thing is that the policy cost goes up with every new city (puppets don't count though, so if you capture a city, always make it a puppet!), so there's a major downside on going ICS. Spoiler alert: you'll find that if you never build more than three cities, it's ridiculously easy to get a cultural victory.
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