As you can imagine the Plummer Project is a massive undertaking. Mayo Clinic has three large campuses located in Jacksonville, FL; Phoenix, AZ; and Rochester, MN, along with many health system locations in Minnesota. My knowledge of everything involved in the Plummer Project is limited because I am not involved the development nor patient care. I do know that if two of your health systems are on EPIC, it should make the communication between those health care facilities and Mayo Clinic easier as any system that uses EPIC can easily share medical records across health care facilities.
I am unsure of how it will affect insurance or the patient portal, but can assure these things have not been overlooked. Mayo Clinic had many experts from every part of Mayo Clinic work on EPIC implementation. If you have specific concerns, I reccomend asking your healthcare provider or contacting Mayo Clinic directly. Also, here is an article from when EPIC went live across Mayo Clinic Health System locations in Minnesota, https://newsnetwork.mayoclinic.org/discussion/mayo-clinic-implements-epic-at-mayo-clinic-health-systems-sites-in-minnesota/.