If you are going to get the FP with MP, then why not buy it now, so you can go ahead and lock in reservations for 10/20 and 10/26 (they are currently open to reserve for both parks, but saturdays in October have been intermittently unavailable for reservations for one park, as you can see in yesterday’s posts here). This way, you will for sure have a park hopper FP with MP for 10/20 and 10/26, AND you will be able to hopefully reserve 10/27 as soon as you scan in on 10/20 ( because then you go down to one reservation for 10/26 only and are eligible to book 10/27). That will still be a full week before 10/27, so it increases your chances of getting that day reserved, versus buying a 5 day PH with MP, then upgrading to FP on 10/26 and trying to get both 10/26 and 10/27, which at that point will be same-day and next-day reservations.
If for whatever reason you decide to do the 5 day PH with MP, and you buy the tickets from Disney with MP already included on the PH when you purchase the ticket, the value of the MP WILL be applied to your FP with MP purchase, per others on this thread. But I still think it makes the most sense to get the FP right now and book those two dates and then hope you can get 10/27 on 10/20.
The only reason I'm holding out on this is because the chance I'll visit during the year again is fairly low, though might happen and will be a bonus.
However, previous years (last four in a row) I've done an October trip down there and that is much more likely for next year. I normally aim to go as close to Halloween as possible, without actually being there during Halloween. Next year I wouldn't mind moving my trip up by one week or so to make use of the FlexPass, but if I buy it now and make reservations, wouldn't it expire around now (September 23th 2020)?