Well, I discovered some ripple effects (no pun intended!) later on in the novel. There's a key scene halfway through, in which the character is a teenager, enjoying a swim in a lake. Of course, I wrote that long before this new change (with the near-drowning memory). It dawned on me tonight -- she's now afraid of the water! She shouldn't be going into the water at all (and if she does, I had better address her strong fears, or else the reader will see it as a major inconsistency).
The difficult thing is that I really don't want to move the lake setting to another dry-land location because I'm satisfied with that scene "as is" (it's an important one, in terms of what happens with her and another character in the water).
I've got a couple of options spinning around in my head, and I'll figure it out one way or another -- but it's amazing to me (and a bit unnerving) how just one small change in a plotline can have such far-reaching ripple effects in the rest of the story. That's why it's essential to carefully glean over the pages after the changes we make, in order to catch these types of inconsistencies.
Not easy, but certainly worth it!