Company swapped insurance recently. Old insurance hadn’t payed off a bill yet, and dumped it on new insurance. New insurance says they won’t cover it, and are asking me to pay. What now?

I had a vasectomy in February. Insurance at the time said they would cover it, and I just had to pay the 25 copay. 2 months ago we swapped insurance, and now that they have taken over, I have a 550 bill from the place that did the procedure. They told me that because I swapped insurances it might not be covered anymore. I said I was told it was all covered and I only owed the copay, but they said cuz new insurance that's not guaranteed. They are trying to get new one to cover it but there's no way to know for sure. What are my options? 550 isn't out of my range I can afford it, but I don't want to if I don't have to.