I got the same notice from my insurance last month, and I already had a documented history of the biosimilar not working for me.
Three years ago, after being on Remicade for about four years, I tried the biosimilar for six months right when it was made widely available, and it wasn't effective. In fact, it caused a flare and it took me a year to get Crohn's back in remission. My doctor wrote a note to the insurance company at that time and they approved Remicade for me since the biosimilar didn't work. I had no trouble switching back to Remicade as far as my gut was concerned. It did take about three infusions to build up to the level where it was effective for me and I got back into remission.
So when I got the most recent letter in December of 2020 about my insurance again kicking me off Remicade, I asked me doctor to write another note to the insurer, which she did, and, once again, I got special permission to be on Remicade due to my history. My insurance will cover Remicade for 2021, and then I'll have to have another note written each year to get the waiver. (This is how my doctor gets to spend her time.)
If you have never tried the biosimilar, you might need to go on it for a while and maybe it will work just as well as Remicade. I wish it had worked for me! It is effective for many people and maybe you'll get lucky.
Before I was allowed to try Remicade, I had to jump through hoops with several other medications that didn't work for me, just to please the insurance company. I've had to live through a lot of flares thanks to the insurance industry wanting to overrule doctors with prescriptions.