I was hoping to get a reply before writing my answer, but I can go forward anyway.
The only type of felony convictions that your state will expunge is certain kinds of drug convictions. Since yours is a Grand Theft, you would have to obtain a pardon, unless you were not convicted of the offense.
There is one other option. It is a long shot, but then all of these post-conviction remedies are longshots. And that would be to retain counsel and have him reopen your old case by way of a petition for the court
to dismiss your charge in the interest of justice
A judge has the power to dismiss a conviction under principles of equity even when the law does specifically provide for that. A judge will use this power very sparingly and generally only to prevent an injustice. You would need to talk to a local criminal trial
lawyer to see whether this is a viable possibility for you. The two of you would have to come up with some compelling reason that the court should ignore the statute under which you were convicted. If your conviction was 47 years ago and you have lived a law abiding life ever since, perhaps a judge will find that compelling enough to dismiss your old case. It's certainly a most impressive achievement.
The DA would weigh in on this, and they would be very likely to object to the petition. If there was a complainant, he or she would be contacted and could oppose this as well. So this is not something you should try to do without a lawyer.
But if you could get a dismissal, even if you cannot get it off of your record at this time, it would still be much better than a conviction.