If you are "hand picked" for a particular aircraft type they simply hire the group a day before a class starts and thus establish your employment date and seniority number. They then only offer that aircraft type to your class to "bid". The next day they start the other pilots and offer them what ever other aircraft slots they want to fill and those pilots get to bid their preference in seniority order, just like you did, except your class only had a single choice available.

Once on board, however, if they open up a new system bid you can bid whatever you want. They may not train you in the new seat for some time, but eventually you will get to go the seat you held on the new system bid.