We've done both before twice. The first time we booked Alaska opening day (in the spring) and then by the time we were onboard next (that fall) the price was so high, it wasn't worth rebooking onboard so we booked a placeholder instead. Then, again, it happened where we had another cruise we moved our placeholder to, then we also wanted to book another. Again, we booked opening day in Feb for summer 2018, then when we were onboard in March, the price was exactly the same this time as opening day (since really, it was only a month later) so we booked the cruise again onboard. At that point, we had two reservations (opening day and onboard). I emailed my TA, she cancelled the opening day one, then she moved my onboard booking to the room I had booked opening day (I wanted that specific room).