Sort of kind of.
The original script was one long script that was (basicly) what we know as episodes IV to VI. The differances being that it only had one Death Star, Obi-Wan survived the first film, and Leia wasn't Luke's sister.
After Lucas decides to only make the first third of that huge script into Episode IV and deal with the other 2/3rds later, he steals the ending from the big script and sticks it on to that one (which is why Episode IV and VI both end with a Death Star battle).
Now as ESB is getting made Lucas still isn't sure how many films he wants to make, and he thinks he might made episodes VII-IX after this trilogy. Leia still isn't defined as Luke's sister (otherwise he wouldn't've kissed her) and the "other" Yoda spoke of would've been somone else that popped up in Episode VII. By the time RotJ pops up he knows for sure that this is the last film, so Leia becomes Luke's sister and we finally meet the Emperor and everything's wrapped up.
Of course then he wants to do the prequel trilogy, but he knows that the effect technology isn't up to spec to what he needs to do it (as he was already developing what would be Corucant for RotJ but scrapped it because of these effects limitations).
So he basically bides his time, doing little experiments into CGI technology with things like the Young Indiana Jones Chronicles and Radioland Murders. Eventually when he sees Jurassic Park he realises that the technology is there to make the prequel trilogy so he starts working on them, with the Special Editions of the Original Trilogy being made both to test out the CGI techniques the prequels would use and to get the Star Wars hype back up.