I'll refer you all back to my earlier post. The system is set up so that each student awarded a navy national scholarship is assigned to a unit in the week or so following selection. Placement will usually place a student at choice #1 from the list of 5 schools, unless that unit has already reached its scholarship cap. If the cap has been reached for #1, Placement will then look to #2, etc., until a student gets their initial placement. This is typically not a concern during the early boards in the fall, but it does pop up routinely in the spring for some of the heavily-requested units. Placement does not at all consider a student's odds of getting into the particular university.
So, after scholarship selection, everyone is placed at a school, whether it was your first choice or not. If you later determine you want to change your placement (didn't get into assigned school, change of heart, family move, flipped a coin, etc.), you have to request a change of placement/wait list request. There are actually two different forms that can be used for this (found on the applicant portal on NETFOCUS), but either way, you'll typically just be thrown on the wait list for the requested school, regardless of their current scholarship loading (determined prior to each year). Yes, you will be placed on the wait list even if there are spots available.
So why does it work that way? The Navy wants to make sure the new scholarship winners from each board (October all the way through April) each have a shot at their first choice school. Basically, Placement looks at you once after selection, places you, and then you stay at the assigned unit until after the placement for the final April board is complete. Yes, you can fill out a change of placement request or a wait list request to join the wait list for another unit, but that request won't be executed until after the last board. That way, Johnny can't jump around from unit to unit while other students (that are still being boarded) haven't yet had the opportunity to receive their initial placement. Yes, on occasion, in certain circumstances, a placement change can happen right away, but the standard practice is to throw the requesting scholarship winner on the wait list.
Once the final board has happened and the final winners have been placed, Placement will handle all the placement changes and move students from the wait lists to the units' assigned lists based on the order in which the requests were submitted (so, submit your request early if your intention is to change placement).
If you have concerns about whether or not you can expect to be able to change your placement to a particular unit, just call the unit and ask. They will know exactly how many slots they have left at any given time. Don't bug Placement with it, as they literally field an absurd number of these requests each season.
Hope that helps. If you need further clarification, please just ask.