No, the hardness is temporary, as a part of growth. 🙂 Fully-developed breasts lose it somewhere during the process.
Also, bras can't "train" the breasts; they affect shape while you're *wearing* them, but the effect is temporary and gone as soon as you take the bra off. (There's some debate over whether bras can prevent - or cause! - sagging, but none at all about whether they can permanently influence breast shape.)