"Fall" season, late Sep to marathon weekend in Jan, is overall a very high
DVC demand time at WDW. Demand during that time makes Easter and summer time look like slow off-seasons. Often during that time, the only things open for multiple day trips at 7-months out are SSR, OKW, and Kidani savanna view, but even those can disappear before 7-months out during the several extremely high demand times in the Fall season, one of which is the Wine & Dine race weekend, which will be October 22-26, 2025, i.e., the main issue you are seeing now is not Halloween time , which is usually not among those extremely high demand times, but that race weekend, which, until this year, was usually in Nov.
Note, you cannot modify any reservation to move just part of the days in a reservation to another resort/room while keeping the rest at the original resort/room , e.g., you cannot modify a 6-night reservation at AKV to 4 nights at SSR while keeping 2 nights at AKV. You would need to modify the entire 6-night reservation to just a 4-night one at SSR and thereafter make an attempt to do a new reservation to get back those first two nights at AKV, which could then be unavailable by going to someone's waitlist when you released those nights.
Otherwise, if you have two separate, but sequential, reservations at one resort in the same room category, MS can merge them into one as long as they are both made with the same membership (all the contracts in a single membership would have the same use year, which I assume is your situation).