OP's situation is interesting... Technically, potential applicant has no clue about any of this and would be entering US with an intent to leave within her authorized stay. If OP proposes and she says yes, then she decides to adjust this would be a true example of changing circumstances. I do not see an issue here, HOWEVER it is likely illegal for OP to be planning this. I do not see how OP's gf would be at fault if she has no clue that OP is going to propose.