The TA should know the number they need to call if they make a mistake on a ticket. It is up to DL whether or not they will fix it for the agency, and there is always a possibility they will not. The TA will have whats called a debit memo against them. I don't remember how all that works, it's been awhile since I've been in Customer Care. But the TA must initiate the call to DL to the special number, not the passenger. The DL agents are correct, there will be no name changes.

In regards to your company with the group that had names put in after the fact. Group bookings are handled completed different. They do allow a set
number of name changes or things to be done that is not in the 'norm' of what
the rules are. So you are correct, they did let you do that before, but the circumstances of the tickets are completely different.

I would take this up with the agency, and be persistent. Just my thoughts!