Hi goingcrazy, don't worry yourself crazy yet! Unfortunately anyone can request a new judge, usually they need a reason (i.e. they know the judge,they have appeared before that judge in an unrelated case,etc.) the good news is, they can do it once, twice would be rare. Anything after that will be laughed at. You or SO should be able to find out why the change was granted. You might have the chance to state the you fear BM requested the change only because she didn't like she didn't like the orders he handed down.
Don't show that you disapprove of the change (it could appear you think you were being favored) If anything you're concerned the new judge may not have had the chance to become familiar with the case (or something to that effect). The biggest thing to remember in court, is ALWAYS make it about the child(ren) and not about BM (unless it's drugs, abuse,etc) Like, " I would really like more time with my child to...."
Try to relax for now, prepare but don't dwell. Good Luck!

I know it sucks,bad. But what ever comes up, make sure you document everything. Even a new judge will see how she is if she keeps changing lawyers. Chances are a new judge will see past BM's crap ,like the other one. IDK what state you're in, or if you may end up in mediation instead of court, or fill in you're lawyer and point out that this is causing a financial hardship, not to mention, the contention is not good for the child. One thing I learned, NEVER start a sentence with the word BUT to a judge or mediator, sounds to confrontational, like disputing what BM says, say what ever would follow the word, just skip the BUT.
Points can be made, if the child always comes hungry, say "he(she) is hungry when I pick (them )up" Not "she never feeds them" Make the point about THE Child, not the BM. Sorry for getting a bit OT.
You have every right to be P_ss_d and more! Use that energy to document and plan for your case. Look up custody laws of your state, you pay good money to a lawyer, but even they don't know everything, you might find something. If nothing else, you'll learn what to expect and the possibilities, and somehow that makes it less scary.