Don't think I'd be happy in the OPs situation tbh; however, I'd not be taking it out on the agent(s).
The ba.com functionality is supposed to allow changes per the OPs circs; and indeed did. The fact it appears to be buggy and can't cope with a 2nd change is IT incompetence not something purposefully not designed (as per open-jaw upgrades being not programmed etc).
Letter to CR, escalated if its met with cut&paste fob-off.