Replacing as a set is a debated subject. Here's what I've found:
Replacing as a set sets the clock, so mean time to fail is known. If they are reaching their useful life, some are not far behind.
Ease of the job. Do them all once, and done. Piece of mind.
Unknown grounding or potential issue with EESS, start with all known goods. Don't mess around here.
If one fails, it could be faulty manufacturing, or Infant mortality. It's just fine to replace one at a time. The A1 troubleshooting guide advises the Ohm method to find them, and advises replacing any one plug that doesn't ohm at 1.5-5.
If one or more is faulty, the total draw on the system drops by approx 11.25 per plug. (another way to see if you have a faulty plug). So measure the amp draw on batteries, properly functioning plugs while cycling should be 90 amp.
So mixed bag on a set..it depends on your records, history of the issue, the job and the issue at hand.
It won't hurt to change out one vs a set.