The simple answer is that if you're there before Sept 6 (Landrush) you'll park on the ball fields or on the paved roads around the grandstand. ...Unless someone changes their mind.
If you've been there, you probably know that nothing is ever certain or consistent; but:
The pre-lineup has been pretty much out on the ball fields for the past couple of years. Back in the olden days they used to just let early-earlies camp in the regular RV park (walnut grove) and "pre-lineup" was only by custom among the early-earlies (and was frequently violated, mostly by local Winnies). Three or 4 years ago the river came up and they decided to move everybody and several were already stuck. (And they found out the county didn't own anything big enough to help move some of the 5th wheelers that were already stuck.)
Last year they had leased the fairground to a group that decided they didn't want to let anyone use the RV area until they'd finished their cleanup; but since they weren't sure they could legally keep people out they demanded $100 per day from anyone who tried to park there. (Only about two people paid it.)
Also in previous years, they nearly always made early-earlies move to lines in the Pecan Grove for lineup; and of course everyone has to move for Landrush.
For the past two years, once you parked you stayed there, and when Landrush came you ran from wherever you landed first. Two years ago there was some problem with people who got into areas where they shouldn't have been, and a few had to move to different lineup positions. Last year, according to unofficial reports and so far as I know, the Mayor of Winfield(?), who "pulled rank" to get into the Walnut Grove early, was about the only one they had to move.
The only "released and official" information is that the only entry open on August 23 (this Thursday) for early arrivals will be the "14th Avenue" entrance, and on/after that date lineup numbers will be given when you enter.
The assumption would be that once the early-earlies are numbered and settled, they'll resume letting people in at the front gate.
Since those who go early will virtually all be there because of competition for the Walnut Grove and Pecan Grove areas, they'll likely try to leave them both completely empty until Land Rush.