Firstly with AA re-issuing a ticket is not the trauma / cost that some of the other OW airlines manage to create. This is routinely done when the date/time of a segment is changed. And this apparently ensures that the other airlines actually are informed of the change(s).

In your case, this is a change that adds the date/time of the segments. If they will issue an eTicket you shouldn't have to visit anyone. And this takes care of all the questions regarding waiting for LHR departures to finalize the changes. As my general rule, making ticketing changes on the day you fly is in the category of "only if you have no other alternatives".

On the question of cost, if I remember correctly there was some limitations to changes to tickets bought in the discount period. Whether the cost difference applies in your case - I would hope not - as this is a category of change for which AA does not normally charge a re-issue fee, ie, date/time changes. Changes of routing would have different rules / fees.

I'm certain others will add their experiences.

Happy wandering

Fred