If you're awarded the 756, you can move to any base that has the 756 without worrying about the seat lock. That is called a lateral bid.

If you're on the 737 or 320, you can bid the 756 at any time after you get your award. The reason being is that the seat lock doesn't prevent you from bidding a higher pay banded aircraft. However, once you get on the 756, you cannot bid down to the 737 or 320 until you satisfy your seat lock.