Well, there are a few things to consider. If you want a specific room type at a specific resort & are willing to pay rack rate to get it, book as soon as you want. If you can only stay at that resort or room type with a discount, there is no benefit to booking it before discounts are offered. Disney releases discounts to increase occupancy. They decide which resorts & rooms need more bookings & those are the ones discounted. For example, at the YC, lagoon view club level rooms are the most expensive rooms. Standard view regular rooms are the least expensive. When discounts come out, you are more likely to get a club level lagoon view room with the discount. Standard view regular rooms are much more limited with the discount or not available at all. Because those room are already booked at the rack rate. If the rooms are already booked, there are no rooms left to discount. You know, just like a retail store, you don't put something on sale if you don't have it to sell. The thought here is typically if you book ahead of time hoping for a discount you may actually be hurting the chances Disney will offer a discount.
Now if you book a room & they do release a discount you want to use: if you book a room only reservation & want to change to a room only discount, they can modify your reservation. If you have a room only reservation & want to change to a package discount ( free dining for example), it must be cancelled and rebooked. Room only reservations & packages are 2 separate booking systems so that's why the difference. If you are hoping to get a package discount & don't want to have to cancel for a discount, you might want to book your original reservation as a package. Then you should be able to just modify if a discount comes out.
And when they try to apply the discount, the resort, room type & length of nights must be available with the discount. There are limited numbers of rooms available with each discount. For example, you have a standard room booked at rack rate. Disney releases 2 standard rooms with a free dining discount. If you are the first or second caller, they can give you the discount. If you are the third caller, you can't get the discount, even tho you already have a room booked.
Sorry, hope all this makes sense. There is a special discount codes & rates subforum here too. You can get lots of good info there as discounts are released.