I have AT&T and just took the sim out of my iPad Air and put it in my iPad Pro and it worked with no problems. The Pro comes with an Apple Sim so you could probably just transfer your plan to it if you wanted but since I have AT&T I didn't want to lock the apple sim to AT&T in case I wanted to use it later.
I did the same thing that @jdusoccer12 essentially did. I tried transferring at first, but was having problems because it wanted me to enter my account email and I only log in with my phone number, so I went to the AT&T, they popped in a new AT&T SIM and transferred my cellular service from my old iPad Air to the iPad Pro. They said there would be an activation fee of $15 and if there is, meh. It probably would be better to transfer the iPad Air SIM but I didn't have it with me because I'm giving it to a family member.
I'm glad I kept the Apple SIM because if I do decide to swap providers in the future, I'll just put the Apple SIM in there and go.