Khakis / Baritone / mc1227,
Putting a temporary replacement's name in Playbill is typically a contractually negotiated term by the agent for the actor coming in as a replacement. But while it's a crafty idea in theory, it's sadly not an effective workaround to avoid having to give refunds in practice.
At the very least, someone who bought a ticket before Dilly and Chase's stints were announced publicly is entitled to a refund. Of that there can be no question. And in point of fact, box offices don't make that distinction (re: when the ticket was purchased). If an above-title star is out, they are required to grant a refund to anyone who demands one (prior to the performance, and assuming it's not a TKTS ticket which has to be refunded at TKTS).
The reality is that Dilly and Chase are functionally still understudies in this case -- high profile, well-paid understudies, to be sure, but still understudies. It's not considered a regular cast change -- case in point: the houseboard, photos and art on the front-of-house continue to bill Matthew and Kelli. As such, if either Matthew or Kelli are out, the regular star absence rules apply and a ticketholder is entitled to a refund, if desired -- regardless of whose name is in the Playbill.
"No matter how much you want the part, never let 'em see you sweat." -- Old Dry Idea commercial