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There is no reason to replace them until they fail. Generally speaking they work or they don't, although secondary windings can fail and produce a moderate misfire the DME is unlikely to set a CEL for.
At the shop I work in we generally recommend replacing all of the coils when a single fails and the mileage is above 75k, simply to save on labor costs. Its cheaper in the long run to visit the job a single time than to deal with repeated failures and full pop labor to revist. We often see coils go out in succession following the first failure.
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