I had an 8+, upgraded to 12PM and was able to keep my same plan. I went into a Verizon store to get it all done as opposed to ordering. I wanted to see the phones in person and I also wanted to talk to a human. Verizon's plans couldn't get any more confusing.
If you are looking for the Ultra Wide Band 5G and live in areas that get service consistently, you would likely have to upgrade your plan. If you are like me and get the "Nationwide 5G" coverage you could keep your plan. Do yourself a favor, disable the 5G Auto and stick with LTE. Better battery life and better speed until the network is further built out.