The DA's Office indicted a defendant for a DWI 1st and the District Court transferred the case to the County Court at Law. The blood results were a .201 and I want to amend the indictment to charge the defendant with the Class A offense of DWI with a BAC > .15. Can I, the County Attorney, amend the indictment, or will I have to dismiss and re-file?

I don't see why not. You're the attorney for the state in the case, regardless of how it got there. Amend away! (subject to CCP 28.10, of course)