Yes, your current character will still be the same in Dawntrail but with updated texture/shader quality/slight model adjustments.

Our understanding is that SE will give a free Fantasia upon Dawntrail release, so that your character can be altered to something that looks good with the new graphics.

When you create a new character or (I think) use a Fantasia, you can select from character appearances you saved previously. However, it may not show the ones made in the Benchmark if the save files weren't copied from the Benchmark to Documents\My Games\Final Fantasy XIV - A Realm Reborn.

Likewise, if you use the benchmark, it is possible that it's not seeing all of the character saves if they aren't in Documents\My Games\FINAL FANTASY XIV - A Realm Reborn (Benchmark).

I think it sees them for me regardless though, without copying them, as long as they are in one of these folders.