You should not need to edit anything ... you told broker A to sell the shares you had in that account. If you had wanted to sell the older stock you should have sold it in the broker A account. FIFO only works if all the stock was in the same account ... and broker's usually follow this rule unless you instruct them otherwise. And in your case you did ... you instructed broker A to sell what was in that account. Changing things now to reflect a better position for your bottom line is begging for an audit. This is why brokers have to keep track of basis and report it on the Sch A ... so there is very little reason to make any corrections except for uncovered sales.
