Does each successful review permanently fix a memory, or does forgetting resume?

Forgetting resumes after each review — but more slowly: every successful retrieval flattens the forgetting curve a bit more.

A single review doesn't freeze a memory forever. After it, retrieval strength starts to decay again — but from a higher baseline and along a gentler slope, because storage strength has grown. Over repeated spaced retrievals the curve flattens progressively, so the intervals between needed reviews can lengthen. This is precisely the pattern spaced-repetition systems are designed to ride: review, let it fade a little, review again, each time buying a longer safe interval.