Direct alteration reprices existing nights, requiring manual edit on extensions

When a host extends a Direct reservation that's already been altered once, the alteration screen reprices the entire stay at today's calendar rates instead of isolating the new night. If current rates for existing nights are lower than what was locked in, the +Accommodation shown ends up less than the calendar rate, and the host has to manually edit Accommodation to charge the correct per-night rate. Example: Calendar showed $161 for the added night, but the preview only added +$101.20 because the existing nights repriced lower. Host had to manually bump Accommodation to get $161 charged. Requested: an option to isolate the added night at the current calendar rate without repricing previously-paid nights. Reported by a Mogul-tier host who noted he's seen this multiple times.