Sorry, didn't catch the word "segment" in your OP. In that case, I would try calling again and note that the only clause in the AAdvantage T&Cs for changes (that I can see) states:
Prior to ticketing, you may change your AAdvantage travel award reservations with no charge incurred. However, if your tickets are reissued after the original ticketing, a charge may apply for each ticket. This service charge is paid at the time your ticket is reissued and is subject to change without notice. The charge does not apply to the reticketing of AAnytimeŽ award tickets. Upgrade, companion and discount award travel is subject to restrictions of fare purchased.
It doesn't say anything about changes after the award commences. If there is any additional documentation for award redemption I don't see it on aa.com.