Is there a legal or ethical problem with someone who used to be a prosecutor defending case which they either filed or indicted while in the DA's office?

Potentially, there is. See Tex. Disciplinary R. Prof'l Conduct 1.10(a). It provides:

Except as law may otherwise expressly permit, a lawyer shall not represent a private client in connection with a matter in which the lawyer participated personally and substantially as a public officer or employee, unless the appropriate government agency consents after consultation.

Any other lawyers in the former prosecutor's firm are also prohibited from representing the private client unless the former prosecutor is Chinese walled and written notice is given to your office "with reasonable promptness". See id. at (b)(1)-(2).

See also Tex. Code Crim. Proc. art. 2.08 "...nor shall they, after they cease to be such officers, be of counsel adversely to the State in any case in which they have been of counsel for the State."