Indictment? What Indictment?

A colleague has a child sexual abuse case set for trial. The indictment alleges one act. The probable cause affidavit attached to the indictment alleges one act. The prosecutor has given notice of extraneous offenses involving the same child which it may seek to introduce evidence of at trial. Then, it gets tricky. Apparently, there may be a question as to whether the particular incident for which the defendant was indicted actually occurred in the county. In answer to this, the prosecutor informed the defense attorney that if that is a problem, he can just elect to prove one of the extraneous offenses which did occur in the county to the jury, and that will suffice. Can this possibly be true? I have heard of cases where the grand jury is presented with evidence that a child was repeatedly molested over a course of months or years, and the grand jury indicts the person on one count. At trial, the defense can force the state to elect which particular incident they are seeking a conviction on, so the jury is required to unanimously agree that one particular incident occurred. That would seem to cure the problem of double jeopardy and jury unanimity. But what about your right to be tried on an indictment returned by the grand jury? In Texas, this is an absolute right. You cannot be charged with a felony by information or complaint unless you waive your right to a grand jury indictment. But, if the state can indict you for one incident, and prove a completely different incident, how does your right to indictment have any meaning whatsoever? Why not just indict someone for "a felony," and then give notice later as to what the offense is, and have a jury trial for that?