Well, what's a "plan"? There's a difference between, thematically, I want these two brothers fighting, versus this is exactly the episode where this happens and then this happens and then then this happens.
The former happens a lot, with vague thematic ideas as to where things will land, but when most people say "plan" I feel as if they're speaking as to solid, concrete flags in the ground of nuance years down the line before a word is even written on a page.
My understanding is that they had a fairly good idea of where everything was going. Obviously not an episode to episode breakdown but a fairly good road map.