Once you've picked a subclass option at level 3, could you later take a different subclass option, or do you lock into a specialty at level 3 and then run with it for the rest of that class' progression?
For example, as a rogue, if you took the "Thief" archetype at level 3, giving you access to Fast Hands and Second Story Work, could you then take the "Assassin" archetype to give you the specific benefits that archetype gives instead of the "Thief" ones?
The PHB rogue description seems to imply that you couldn't:
At 3rd level, you choose an archetype that you emulate in the exercise of your rogue abilities: Thief, detailed at the end of the class description, or one from another source. Your archetype choice grants you features at 3rd level and then again at 9th, 13th, and 17th level.
but I'd like a more certain answer, especially one that refers to archetypes in general. Also I'm not ashamed to admit I'd prefer a source with some authority that tells me you can mix and match, but I am looking for the truth of the rules when all is said and done...
Different to Can you multiclass the same class twice for different class features because the no on that demonstrates that you can't re-take a level of the same class. Taking different features for different levels is not the same thing, and would work differently.
Similar to Can I mix monk traditions? but that one was specific to monks - which is why it didn't show up on any searches I did, and why it wouldn't on the searches of others. This question is specific enough to be clear and precise, but broad enough to be useful to others who aren't in the exact same situation/using the same class.