page 72 of APG says that I can take multiple archetypes as long as they don't alter the same class features. A Paladin can not be a hospitaler and an undead scourge because they both modify smite evil and replace aura of justice. But s/he can be an undead scourge and a warrior of the holy light since none of their new class features replace the same core class features. How exactly does this work? Also what about starting as an archetype for a few levels then switching to core class for the rest of the progression. I've looked on the boards and I can't find any answers to these questions. I'm very interested in starting as a beast master ranger for four levels than switching to core ranger before I lose camoflauge. Can this be done?
You just apply both archetypes. Simple as that. There is nothing fancy or complex about it.
And concerning your second question (which you should have separated into a paragraph or something), there aren't really rules for that yet. Ultimate Campaign, an upcoming hardcover, will have rules concerning retraining, and that will hopefully cover retraining archetypes.
On the other hand, that totally feels meta- and powergamey. I probably wouldn't let you do it if I were your GM.
When you take an archetype, it's all or nothing. And since you can't take a class you already have levels in, you can't multiclass as two different paladins with conflicting archetypes. You can't even multiclass as "alternate" classes (ie. rogue/ninja) because they're still considered the same class; just kind of like an uber-archetype. Someone linked a page that listed every archetype and what it will mesh with but I'll have to dig it up. Or, to put it another way, you'll have to dig it up. I'm hungry.