Except modifying a reservation (online or contacting IHG/Hotel) is always under the covers a "Cancel & Rebook at the on the day rates and terms" (eg things like minimum stay rate if make longer/shorter stay. The only modify about the process is tht the IHG Reservation Number remains the same

So if you sucessfully mod dates of reservation, the new rates as notified when modding apply to your stay, not the original no longer booked rates in the email sent when you first booked

This is why if you book say 5nights stay, 3months out at real low flex rates whilst hotel has many rooms left, and decide 5days before stay you only need first 4 of 5 nights, and hope to save $$'s modding res to just 4nights, you often find the cancel+rebook 4days 5days out when hotel now high occupancy, actually costs more rather than saves you money.

Only option to reduce 5->4nights is at checkin to mention to checkin clerk you now only need first 4 nights. Often clerks simply remove said last night, droping off final night at that nights booked rate, rather than reprice. They are not meant to, and it annoys the hotel revenue managers, but with a queue of guests, they just want to get you checked in and move on to next guest.
Also harder to do this now, as these days many/most hotels flex rates are cancel by "day before checkin". Luckily majority (not all) of corp rate staysare still cancel to day of checkin.

Alas not a possible trick if you need drop first night of multinight stay. On longer stays, I often book first/last nights as individual res's so easy to drop a first/last night if travel plan changes, and avoid costly repricing