Well, the FAQ on the first page of the thread answered it, LOLYes you answered it. So you do have to scan in to Epcot.
But if MK was your first park and you didn’t scan in to Mk you would have to go to Guest relations and change the park pass I assume. With flight delays/cancellations/park passes selling out I’m sure people will encounter this.
Thankfully, park passes selling out is not a common/daily thing. Most of the time, you can swap on the fly
Likely advice would be avoid Epcot as your arrival day park if at all possible.
Then you don't have to worry about that part at least. Remove as many possible hurdles as you can.