I kind of think you are overthinking this.
Has your child gotten a pre-read from Admissions and passed?
Has the coach said “You have a spot on the team, and I will support your application with Admissions”?
If the answer to those questions is “yes” then you can be as comfortable as anyone can be that your child will be admitted.
(Well, unless the school is MIT or similar where coach pull is not very strong. But you said there was 98% certainty, which is a more typical D3 athletic recruiting scenario)
If the likely letter doesn’t appear by October 15, then it could mean there was some explainable delay, in which case, no you would not want to change the ED application right? Or, could it mean the coach suddenly pulled support? Without telling you? I believe that is incredibly, incredibly rare. In that scenario if you were to switch to another school for ED, would your child be able to get coach support at that second school? It seems (without knowing the facts obviously) pretty unlikely because that second coach has probably used up all his spots with Admissions already.
Is the first school your child’s first choice even without the sport (as people recommend)? If so, why not keep the first application ED?
Anyway, probably all of the above is unnecessary – if the coach has offered a spot on the team and has said s/he’d support the application with admissions, I would not spend a lot of energy planning out a strategy for an extremely unlikely outcome.