I have tried Googling all kinds of wording and a lot of the results are outdated.
Most discuss the lvl25 Boost still being available and whether the incoming lvl40 Boost would unlock Subclass nodes.
All the official descriptions mention is the fact that the lvl Boost can only be used on a Character [b]below[/b] the lvl the Boost will reach, and the Subclass boost only working with a [b]blank[/b] or [b]partially unlocked[/b] Subclass.
I took some -Bl00-D4M-ING- baaaad advice from another player to boost a Hunter Subclass for PvP rather than attempting to boost my new Warlock Subclass when I unlocked it.
[spoiler]Basically I'm feeling pretty blue about it and rather sheepish when it comes to paying for another booster pack in case it doesn't let me.[/spoiler]
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Yes. If you wanted to, you could buy 9 subclass boosters and max all 3 classes subclasses. But honestly, you could just do each story mission/quest and max your subclass before finishing the vanilla missions.
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Just play the game.. You'll be surprised at how fast you can level up a character / subclass without the need for a booster.
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Don't use a level boost. All you have to do is level your class to level 40, then go to your main and do archons forge or strikes. Any leftover junk you get as rewards that are a couple of light levels under you can store in the vault. Do your rise of iron book and try to get the days of iron armor. Each package has a piece of armor for every class and you can give your class the days of iron armor. Then you take the junk you got and give it to the class you level up and you infuse your classes days of iron armor with that junk. I got from 303 on my titan to 370 in 10 mins.