Do I need to download the client again?

I have read here that Guild Wars 2 client is available.

I have pre-purchased the game, and I have been participating on the beta weekend events, so I have already downloaded a client from them.

However, I'm not sure if the client for the final release is different from the one for the BWEs.

Will I need to download it all again? (I hope not, it's like 16GB)

You can continue to use the beta client after the launch. You do not need to download again. However it should be noted that some people have reported improvements in the games performance with a fresh install, this is purely anecdotal and does not hold true for everyone.

If you prefer to have the game 'installed' properly, you can download the installer, run it, and once it begins downloading the game again click cancel. You can then copy the .dat file from your current beta install into the new game folder. This should prevent the need to install the game while giving you things such as a start menu folder e.t.c. I have not found any evidence that this will increase the performance of the game client. This also only applies to those you downloaded the very first BWE client, which used a different launcher style and simply downloaded the files to whatever directory the installer was located in.