Do airlines ever downgrade first class passengers to economy?

I have never flown first class before. Always wanted to at least give it a try. Back in April of this year, I booked my flight to see friends over the holidays in December. Back in April, the first class seats were being sold for only about $150 more than economy seats. So I booked first class.

Now that my flight is only about 3 weeks away, I was curious as to how much the first class tickets were. Delta says there are 3 first class seats available for $1500 each. But when I click on the seat map, all of the first class seats are taken.
I don't have any special status with Delta. Will I be downgraded back to economy? Is there any compensation if I'm?
I've been looking forward to experiencing this flight for months just to cross it off my bucket list. I would hate to be sent to the back of the plane, especially after paying extra to sit up front.

Added (1). Maybe I wasn't clear, but there are 12 first class seats on this A319 aircraft. All 12 seats are taken… And they are still selling 3 more seats. How can they sell 15 seats if only 12 seats exist?

It is highly unlikely they would downgrade your ticket. The reason they are more expensive now is that it is closer to travel time, and it's around a holiday, times when prices skyrocket.

No, you won't be downgraded if you bought a First Class ticket.

If you have a contract with an airline to fly first class, even if you got it at discounted price, the airline is in breach of its contract if it does not fly you per your agreed contract. You don't have any cause for complaint until you are actually told you have to fly economy. If that happens, you say that this is in breach of your contract and that you will be taking legal advice. Do not get loud and brawling. It does not help. Be civilized because you have the upper hand in this. And my feeling is that your seat is probably already allocated to you. There's no unseemly scrabbling for seats in first class.

It has been said that first class pays for the plane so the coach passengers can tag along. So, no.

The reason that they look like they are all occupied but there's the chance to purchase them also, is because they are currently occupied by individuals with airline reward status.

These individuals will be down graded if the remaining first class tickets are purchased.

All this said, I wish you would have asked about the first class experience before booking. Different airlines handle it slightly different but in general:
- First class seats will always be better than coach. You will usually get a nice blanket, pillow and some special magazines.
- Flight is less than 1.5 hours, the only difference between coach & first class is you can get a couple of free drinks. There will unlikely be meal service
- Flights between 1.5-3.5 hours (in the air), you will get a meal (choice of 2 options unless they run out) but it won't be anything super special. Unless the flight takes off after 8pm when you get more of a snack.
- Cross country flights often come with much better first class services - chef prepared meals, more selection, prepared dessert.

Unless you are doing a cross country flight, first class/business class isn't all that special.

No.

Do airlines ever downgrade first class passengers to economy?
YES it sometimes happens.

The Airline attempts to occupy ALL Seats on the plane.

Some passengers may get an upgrade to First class as a reward for some reason. This allows their previously booked seat to be sold to someone else.

Should a Cash paying First Class passenger come along the "reward" customer is placed back in his old seat.

Your three seats available just indicates the Airline still has three seats they could sell.

If they do not sell they have assigned them to some other passenger on the flight.

Delta like all Airlines has these things called HUMANS you can talk to one about your situation.

Computer booking site is not set up to answer every question you can dream up.

It is extremely rare if this does happen. Do you have anything to worry about? No. As far as why it it appears to be sold out, sometimes an airline will hold a couple of seats for the reward members for an upgrade. For red eye flights, the pilots usually rotate so they aren't getting tired flying the whole time (depends on the flight, not all flights are like this).

A. The seat maps are not 100% accurate. Airlines routinely block seats on the map for various reasons.
B. Fares go up closer to departure. This does not affect previously purchased tickets
C. While downgrades can happen, its very rare. It's not an every day occurance.

Overselling seats is common practice, it ensure the highest load for each flight. Do they compensate for running out of seats in a cabin, yes, but almost certainly they expect some of those booked to not turn up, and book later flights on a fully flexible ticket.