Is it permissible to re-ask or reconsider a previous fatwa from the same mufti who issued a ruling of impermissibility, due to the possibility of the fatwa changing?

There is no harm in re-asking the same Mufti, or asking another Mufti regarding issues of scholarly difference. An ordinary person acts upon the opinion of whomever they trust. However, it is better for the ordinary person to limit themselves to asking one scholar whom they trust, so as not to fall into confusion. One's obligation is discharged by asking a single scholar, and they are not required to re-ask every time a new situation arises.