Hi Jessica,
I have not found an issue with face changes from my passport picture, both local TSA as well as foreign inspection systems are very forgiving. Yes, gender incongruities may require pat downs but those have been discrete and not a problem for me.
I have never tried to change just my passport picture but just recently both my partner and I have gotten passport updates as well as a new one for me with both name and gender change. My partners passport renewal took one to two weeks, arriving here two weeks ago. My totally new passport with name and gender changes took two to three weeks getting here last week; both of those in the Christmas crazy season. If you desired a change in passport, you should be able to get it done well in advance of your late May travel date.
Enjoy your travels and good luck chasing your ancestry
Tia Anne