I am going to tag @CobaltDragon in this, because it might be a better chance he may see your post and pop in with an official answer. According to the main announcement page "... when you hit certain level breakpoints with lancer and brawler characters, you’ll receive special leveling rewards..." I can't find anything that specifies whether it is only for new characters, or if the event includes existing characters. Because of that, I would "guess" that probably any Lancer or Brawler who is not yet level 64 can get the rewards, new or old.

If you have an existing Brawler or Lancer, then I just recommend you try to go up a few levels and just see if you get some rewards in your Parcel Post, for that new level. There are rewards every 4 to 7 levels (depending on which level you are), so it shouldn't take very long just to hit one level with rewards, to see if you get them. If you get rewards, then I STRONGLY RECOMMEND going on and getting the other rewards. This event is great, and the free rewards are amazing. I actually created a Lancer and a Brawler on my main gamertag, and got them both to level 65. I had so much fun, I actually created a Lancer and Brawler on my alt gamertag and got both of them to level 64. I will go back get those two up to 65 later, but 64 gave me all of the rewards from the event, so I took a break for now. :)

If you want to just try your existing character, then check this list for all of the rewards, at which levels. https://forums.enmasse.com/tera/discussion/29664/tank-leveling-event-for-lancers-and-brawlers Just look at the next level up from your character's current level, and shoot for that level. When you hit that level, you will get rewards in your Parcel Post, if the event applies to existing characters. You can only do it for ONE Brawler and ONE Lancer per server, so be sure you don't plan on creating that same type of character for the event, if you start leveling up an existing character for the event. Whether you want to try leveling a current character (to see if it works), or if you want to create a new character, be sure to do it soon. The event ends on Friday (23 November).

This is accurate. :)