I have done the reallocation of SO for an SO exchange made within the 8 month window as well but not after the addition of a new purchase. When I asked to deposit a week from a specific resort they initially told me the SO for that unit had been used for another reservation. After some gentle persuasion they went into the system and rearranged things the way I needed.
I do not believe it will be an issue for the OP either. You may have to do some "gentle persuasion" as well, but it can be done.
I understand the OP is referring to a SO only ("internal") booking but I believe the new(er) II inventory management system may help her with achieving her goal as it makes it that much easier for SVO to "stir the pot" to reallocate SO. This, along with the ability to "Pull" deposits back from II and to deposit late without losing trading power are the few advantages I see in the new(er) system.