Switching formula am I allowed?!

So my baby was born prematurely and is having issues with similac Neosure. Her pediatrician don’t want her switched. Am I allowed to switch her anyways like do I HAVE to have the okay from her doctor? She’s constantly spitting up and fussy. Super gassy and constipated. I burp her lots, keep her up after feeds for an hour, give her the meds they prescribed for the symptoms (only the constipation meds work). It’s so stressful that she’s so uncomfortable all the time and spitting up on everything constantly. And she chokes on her spit up so I’m getting no sleep because I’m scared I won’t know if she’s choking. I went twice about the issues and still won’t let me switch her.