Tefillah is a request, יהי רצון “May it be Hashem’s will” that we have healing, health, or whatever it is that we need.
Yes, we can ask Hashem to change His will.
Hashem’s Ratzon/Will is like a “garment”; it doesn’t change the essence of the one wearing it. Our life, our future and all that happens to us is the result of the specific will and “garment” with which Hashem chooses to clothe Himself as He takes care of us. (There are many reasons for Hashem’s choices of garment. These include but are not limited to our behavior and our conduct, as well as each person’s individual purpose for which our soul comes down to this physical world.)
When we daven, we pray that Hashem should have a new will, a “new garment”, and grant us whatever it is we are asking for.
Your tefillos have the power to change anything negative to positive. Pain to pleasure. In a revealed good way.