If I am understanding it correctly, with a Dec use year, you currently have an SSR reservation in a standard view 2BR using 275 points for a week in Nov, which does not include the higher point Wed to Fri of Thanksgiving week (Nov 27-29), meaning your Nov week ends before then.
You would like to change that reservation at 7-months out to BWV for a week in a boardwalk view or pool/garden view 2BR which would cost 327 points, but to make that change at 7-months out, you would need to borrow the 29 points from your Dec 2024 use year and purchase 23 OTUPs. To make that type of reservation change, you would need to wait until 9 a.m. eastern at 7-months out to contact MS because of the need for OTUPs. You are correct to assume that your ability to make that change an hour after the 7-month window opens is highly unlikely. Be aware, that even if you could attempt it at 8 a.m. at 7 months out, your chances are extremely low -- BWV studios and 2BRs (all three booking categories) are usually unavailable at 7-months out in Nov.
You also mention that you should have tried for end of Nov and then first week of Dec, but be aware that first week of Dec is the highest
DVC demand week of the year at WDW and being able to get a a 2BR at any near park resort at 7-months out will potentially be impossible -- even SSR 2BRs can fill before 7-months out for that week. Moreover, you would have end of the use year complications. You cannot online do a single reservation that overlaps a use year, and thus needs to use points from two different use years. You would have to do two separate reservations online, one for the Nov time and then later one for Dec time, or call MS to do a single reservation (which cannot be done until 9 a.m.)
My sense is that it is best to keep what you have and see if you can get extremely lucky and make a switch at 7-months out. Trying to rent your SSR reservation and then also attempt to purchase a rental of a BWV reservation, would, even if successful, likely mean you would lose money because the amount you can get for an SSR reservation is likely less than what you have to pay for a like BWV reservation even if you can find someone to do one.