I think the CSR was wrong on this one.
What DL offers is risk-free cancellation, where you can cancel with a day (midnight the next day specifically) of booking your ticket. You pay for it, it gets charged, you cancel, the refund goes onto your card within about 24-48 hours on Delta (normally with NW it's taken around 72-96 hours to post, but it does).
As for fares changing, they do all the time. Every airline changes fares. There have been times I've kicked myself in the rear for not jumping on a low fare and ended up paying significantly more (such as passing on a UA $160 flight to LAS over Memorial Day while I holded out to see if DL would offer Double EQMs - they did, but not on the cheap fares and I ended up having to pay for a flight that gets the double EQMs, just at double the cost...).