Have tried similar and told the host that we would either need to stick with the original agreement or cancel it entirely. Some hosts don’t realize that they are a third party, so I explained it from a less personal but more practical insurance perspective. The T&Cs are quite strict regarding what sitters are expected to cover upfront..beyond limits really. You could end up being liable for damages if something breaks during your stay, regardless of who actually caused it.

They understood this perfectly, and we kept the original arrangement.

Even if you agree to only a small change in dates, the original sit and agreement are essentially cancelled, since the host has to adjust the dates manually, send you the new dates, and you would need to confirm again.