Yes, changing your final destination would count as a change in O/D.
By the way, you're allowed both a stopover and an open jaw. So, think about your plan before you book and pay for your change.
For example, you can go IAH-FRA-VIE, VIE-FRA, FRA-IAH, using your stopover, not open jaw.
Or, if there's a different city to continue to, you could do that, too -- e.g., IAH-FRA-VIE, VIE-MUC, FRA-IAH, using both your stopover and your open jaw.
And so on.
And don't forget to allow some FCT time in FRA, preferably on your return.