Q:

Was lightsaber physics planned from the start, or was it retconned later, possibly due to a "Discovery" episode?

Today apparently I'm being inspired by other answers. 
In Star Wars, is there a pretend-science explanation of how lightsabers work (like midi-chlorians for The Force)? describes that the pseudoscience behind lightsabers is plasma, contained by a special field, etc. The comments point to a Discovery episode, etc. 
Here's the thing. I don't recall anything from the films talking about plasma in lightsabers, and I know that many of us who were fans when the original trilogy was released in theaters we're like, "cool, laser sword. Why not?". 
So I'm wondering if there is any evidence that George actually figured out on his own that laser swords wouldn't work, and planned on making them plasma swords from the start, or if it's been retconned in, possibly as a result of that very Discovery episode?

A:

The scripts for both the original trilogy and the prequels refer to lightsabers as "laser swords". The Episode IV script uses the phrase "laser sword" three times, so Lucas evidently thought of lightsabers as laser-based from the beginning. For example, the script says:

With astounding agility old Ben's laser sword sparks to life and in a flash an arm lies on the floor.

The phrase "laser sword" is also spoken by a character at least once, by young Anakin Skywalker in Episode I:

ANAKIN : ...you're a Jedi Knight, aren't you?
QUI-GON : What makes you think that?
ANAKIN : I saw your laser sword. Only Jedi carry that kind of weapon.
Episode I script

This can be explained away in-universe as an ignorant young slave misunderstanding the physics of lightsabers, though it's a more prominent example of how Lucas interpreted lightsabers.
The Episode III script, which was the last film under Lucas' control, uses the phrase seven times. For example:

ANAKIN ignites his laser sword.

The phrase "laser sword" is not used in the Episode VII script. Episode VII was released under Disney, so evidently Disney is avoiding the "laser sword" description.
Similarly, blasters were originally described as lasers but have been retconned as beams of plasma in the Disney canon novel Lords of the Sith (the Episode VII script refers to  blaster fire as lasers, though, so Disney's retcon isn't consistent). It seems that Lucas thought of both lightsabers and blasters as laser-based but Disney is changing them to be plasma-based.
Note that plasma-based lightsabers were introduced in the old Expanded Universe (now branded "Legends" by Disney). Wookieepedia's Legends article on lightsabers describes the lightsaber as plasma-based and cites the Legends novel Luke Skywalker and the Shadows of Mindor, which was released in 2008 (before Disney took over). It's unlikely that EU/Legends authors got the idea of a plasma-based lightsaber from Lucas since Lucas claimed he didn't know much about the EU:

When asked in an interview his general opinion on the EU, he replied:

"I don't read that stuff. I haven't read any of the novels. I don't know anything about that world. That's a different world than my world. But I do try to keep it consistent. The way I do it now is they have a Star Wars Encyclopedia. So if I come up with a name or something else, I look it up and see if it has already been used. When I said [other people] could make their own Star Wars stories, we decided that, like Star Trek, we would have two universes: My universe and then this other one. They try to make their universe as consistent with mine as possible, but obviously they get enthusiastic and want to go off in other directions."
    ―George Lucas, from an interview in Starlog #337

