Yup. It does give a false sign to Disney and in fact ... that false signal effectively lowers the number of potential "fall promo" rooms at the resort you picked on the dates you picked. And if you picked a popular resort and other people are doing the same thing (and they are) ... it gets even harder to secure your dream resort for your fall trip on the dates you picked.
Thus, making it harder for you to switch to whatever promo comes available. They are different room inventories. Therefore .. when you drop your room only booking and switch to the promo.. Disney is left holding the bag on a room which is NOT part of the promo and which they thought was previously booked.
Why should you care about that? Because to the bean counters eyes - that exact behavior reduces the effectiveness of the promo when all is said and done. Less effective promos are dropped and not repeated in favor of promos that are more effective to the bean counters. They don't see all the 'changes' to existing bookings in gross detail. They see room counts. X amount of promo rooms booked and X amount of non-promo rooms booked. And if the non-promo number goes down and the promo number goes up by an almost exact amount. Well duh, that's not an effective promo.
See where I'm going there?
And speaking of bean counters.. every contact you have with Disney on the phone costs money. It costs Disney money and (eventually) costs you money. If call center costs go up ... guess what? Room prices will go up to offset that and ensure the profits etc. So you may still get a 30% room discount ... but it will come off a much more expensive room each year.
Just having a booking now guarantees you NOTHING regarding a future promo.
Nothing.
If you are able to convert your reservation -- then you would have had just as much luck booking an entirely new reservation ...
Nobody gets this. But I'm going to keep repeating it anyway.