I'd guess that something happened to the real Char so that Casval took his name.
I also wonder what's up with their ages. In 0068, Artesia was 6 (because she was 17 in 0079) and Casval was 11. After a timeskip and during their stay on Earth Artesia was 9 and Casval was 14. I though that they'd migrate to Texas Colony soon after the attack, but apparently it took 2 more years? Well, maybe it took some time for Mr. Teabolo to recover and arrange everything.
If so, it makes Amuro 9 y.o., I guess.