If the HTC One M8 has the ability to make VoLTE calls with Advanced Calling feature enabled on the phone, then I think you should be okay.
If the phone won't work after the 3G shutdown, Verizon Wireless should be reaching out to all the customers beforehand to inform the device will need to be replaced
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
