You could have always upgraded your phone without affecting the grandfathered unlimited data plan and hotspot feature. You had the option to purchase a new phone at the full retail price and most recently you have the option to purchase using the Device Payment Plan (DPP) agreement to spread the cost of the phone over 24 months.
As for the new Verizon Unlimited Plan, the hotspot feature is included in the data plan without an extra charge. You are allowed 10 GB of LTE high speed hotspot data before you are slowed down to 3G speeds per billing cycle per line.
Personally I was in the same position as you with keeping the grandfathered unlimited data plan and purchasing a new phone. I opted for the DPP when Verizon Wireless had a decent deal over Black Friday weekend. It didn't affect my plan or options. I also had to consider whether or not to switch to the new Verizon Unlimited Plan when it became available. I had to look at the cost and how much data I used to determine if switching was right for me. What worked for me might not work for you.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.