Keep checking your seat assignements, especially for the emergency exit rows
 

Not sure how many people still have status, but here's a PSA if you booked seats in the emergency exit row (which I highly recommend if you can get them for free with your status). Frontier is CONSTANTLY subbing out their aircraft. If you book the emergency exit row, there is a very good chance that your seat assignment will not be in the emergency exit row (and you won't get the extra leg room) when you fly. Frontier does NOTHING to notify you of this, and doesn't reassign you to the actual emergency exit row. You need to do this yourself. I'd check my seat assignments every 2 or 3 weeks if you've booked ahead, and more often in the last week or two. Sad, but it is what it is.

Yeah it's happened to me a number of times so I book row 3 instead to insure extra space. Problem is when it happened last two times it was a same day sub out. Or at least they didn't inform me in any way on the app. I checked about 3 hours before the flight and it all appeared fine and then got on the plane and the exit row was 3 rows away. I explained it to the FA and she said it was a pretty quiet flight so go ahead and switch but that was lucky. The flight before I lost it I checked maybe a week before and didn't see any issues so I assumed it was ok. Went through the seat selection on check in and it still seemed ok. Got on the plane and it was not and it was a full flight. Not too happy about it but that's flying for you.

Yes, this happened to me while boarding a couple years ago, and now I'm more careful about it. Which doesn't mean it couldn't happen to me the way you describe.

The "problem" with row 3 is that most elites prefer that row to the emergency exit rows, so it's more likely to be full. And, of course, there's more legroom in the emergency exit rows. But there is this plane-change risk!

I've had this happen to me too. I noticed it in Denver before I got on the flight, so I went to the Frontier service counter, which of course, did not help. And neither did speaking with the gate agent or an FA.

Were the actual emergency exit row seats taken by the time you realized it? Because they should be happy to put you back in the emergency exit row if it's available. If it's not available and you get hosed, I'd contact their customer service and see if you could get a voucher from them. I've not had to do that, so I don't know what they'd do.

I had this happen about a month ago. And I was flying with a colleague, who had never flown Frontier before. I was like, it won't be bad, I've got the exit row for us. And then.... nope.