Code shared flights after BA / Aer Lingus merger

Merger or not, airlines don't switch people around unless badly necessary (IRROPS -- weather, plane mechanical failure etc). If they cancel the route entirely it's a different story but such radical changes rarely happen this fast.

Short answer: nearly no effect on the service.

For some cases, airlines merge means that only 1 airline brand exists. United & Continental, American and US airways are the examples. The aircraft, reservation and all others things are combined into same brand names.

On the other hand, some airlines will keep the subsidiaries' brand even when they totally take over another airlines, like Dragon Air (Cathay's subsidiary), IAG group (Parent company of BA & Iberia) and Air France-KLM. Although they have different brand name but they may combine their reserve system and other equipment, they may also codeshare their flight.

As the CEO of IAG said in May "Aer Lingus would maintain control of its brand and operation while gaining strength as part of a profitable and sustainable airline group in an industry that’s consolidating", Aer Lingus will maintain its own brandname even after it has been takeovered. Moreover, the part of the deal between IAG and Irsh government (25% Aer Lingus share) required not to decrease services between London and Ireland + North Ireland and increase transatlantic services.

Given the fact that BA wont forgo Aer Lingus brandname, and they wont reduce Aer Lingus transatlantic services in near future, plus the fact that BA wont fly between Dulles-Dublin , the services wont be affected in the nearly future.

PS: British Airways is not in the process of buying Aer Lingus, IAG has already bought Aer Lingus (98.05% share)

British Airways is neither buying nor merging with Aer Lingus.

British Airways' parent holding company (a Spanish company called International Consolidated Airlines Group S.A. or IAG) has bought and delisted Aer Lingus. IAG already owns two other airlines (Iberia and Vueling) and is in the market for more, although this latest acquisition is going to take a lot of management time.

Your existing contractual arrangements with United and Aer Lingus will not be affected.

Ultimately, and subject to regulatory approval, IAG will be ending Aer Lingus's association with United and bringing it into their transatlantic joint business venture, which is a kind of cartel with American Airlines, British Airways, Iberia and Finnair, who coordinate prices and schedules on services between Europe and North America, and share their revenue. This will take at least a year because of major IT deficiencies at Aer Lingus and the complexity of the legal landscape. The Aer Lingus brand will be retained, indeed the plan is for a major expansion with Dublin being turned into a major transatlantic hub. Aer Lingus is not being subsumed by British Airways (nor by either of the other IAG-owned airlines).

However, to reiterate, although those codeshares on United will eventually disappear, this will not affect any existing tickets, save perhaps for making involuntary changes (where if anything you will have more options rather than fewer).