Then the agent is correct... for the SDC you'll need availability in the same fare basis. The offer of a change fee-waived buy-up to D is proper under those circumstances.

Leveling at 24h was the case at one point, but it has tightened over the years. These days, anecdotally, if a flight is Y9 or so at the 24h mark, I tend to see leveling of fare buckets to W or so, sometimes down into the discount fares. Inside of 3h it seems more reliable across all fare buckets, but not always the case.