Once you purchase GW2, you own it. There is no purchasing it again for "real" play vs. beta play. ArenaNet was just thanking their supporters and letting us avid gamers help them test it and find all the bugs that we do all the time anyway so they could give us the best gameplay experience at launch.
If you still plan to purchase the game prior to the release, I would do it sooner rather than later. The 3 day headstart will still apply up until the launch I believe, so any fully purchased game will qualify. The game is large and will take some time to download, so you want to do it soon so you aren't waiting for the download and wasting game time on that.
For those who don't know, www.guildwars2.com has a countdown timer to the 3 day headstart, which starts at noon Pacific time on Saturday, August 25th. I'd personally make sure I had the game purchased no later than the 20th so that if I had any issues with registering and/or download (not that I've heard of many) they would be resolved in plenty of time and I'd be ready to go right at noon that day.
Good luck and hope to see you in GW2!!
16 days and counting til we can play, all day, any day ~ Woo HOO!!!