What are the chances that my bones will drift back to pre operative position?

More commonly it is the
cartilage that has “memory” and is a bigger problem with return to the
undesirable direction. If at 6 months your nose has kept straight, you
can take a breath in the confidence that is not likely to shift any further.

  At 6 months, the bones have healed, and they should not drift.Kenneth Hughes, MDLos Angeles, California

The nasal bones are well set at 6 months after the rhinoplasty. They're usually very well set at 6 weeks after the rhinoplasty procedure and should not drift at all, unless  you get hit in the nose

At six months after surgery, the cartilage and bone are well healed.   Accordingly, unless you break your nose, the changes should hold up quite well.