Surprise dental bill after 2.5 years

My orthodontist’s practice was recently taken over by a larger practice and their billing department reached out to me saying I have an outstanding balance from over 2.5 years ago (Feb 2021) because my dental insurance was terminated 3 months after my treatment began and they couldn’t get reimbursed in time. They said I need to pay what would have been the amount reimbursed by insurance or it will go to collections.

At the time when I started treatment, I paid the full invoice (without insurance payment) and thought that their office was reimbursed because I didn’t receive any communication about it until now. I was still corresponding with the orthodontist’s office over email until end of last year and they never sent or communicated anything to me about an outstanding balance. They apparently did send one invoice to my old address in November 2023 but I had already moved by then.

I’m feeling frustrated as I don’t have that amount of money to pay at the moment nor do I feel it’s 100% on me to pay since it wasn’t communicated in a reasonable amount of time. I wish they just had told me sooner because at least I could have prepared for this better. I also ended up getting new dental insurance during the treatment period and I could have used that insurance to resubmit the claim.

Does anyone know ways to dispute this or is this a lost cause at this point? I know a little more about how medical billing and insurance works but more clueless on the dental side.