May an airline change the aircraft from the scheduled one?

You’re in luck. Last time I flew from Alicante the airline had forgotten to schedule one (yes, they really had failed to plan for an aircraft to be there for the trip) and aircraft and had to rent one from another company in order to get us from there to Amsterdam.
Result was a 6 hour delay while an aircraft was sought and found from a wet lease agent in Denmark, flown to Amsterdam to receive Transavia cabin crew, then flown to Alicante where it had to wait for a gate slot before we could board for the trip.

I’d rather not have had a rule that they’re not allowed to use an aircraft not scheduled for that specific leg or I’d still be stuck at that airport several years later.

Or the time in Amsterdam when the scheduled aircraft was struck with mechanical trouble in Malaga and we had to wait several hours for another one to take us out. The one that went tech was grounded for several days…

Same happened in London once, got lucky BA had another aircraft ready to take us to Amsterdam from there that just needed towing to the gate and fueling up.

Still want a law that prohibits airlines from using aircraft other than the specific specimen originally scheduled to fly a leg?

For people who choose a specific flight and time based on the scheduled aircraft it can sometimes be a disappointment, but such people are far less common than those who choose a specific flight based on its timing and destination.
And of course many carriers won’t disclose the specific aircraft scheduled until shortly before departure (though they usually do list the scheduled type on their schedules).

Yes, airlines can and do change the aircraft to be used for a particular flight (often referred to as an ‘equipment change’) for a number of reasons, including, but not limited to:

• The scheduled plane is down for maintenance.
• The scheduled plane is delayed in arriving from another airport.
• Some need came up for the scheduled aircraft to be used for something else.
• Their expectations on how many seats would sell were not met, so a different aircraft size ended up being more appropriate for the given passenger load. This usually doesn’t occur at the last minute, but it can happen long after you book your ticket.

None of these generally entitle you to a refund or even to keep the same type of seat that you originally had (aisle/window/middle, front vs. back, etc.,) though they generally will try to make it something as similar as reasonably possible. The only situation I can think of where this might entitle you to some kind of compensation or refund is if you booked your ticket in a higher class and they did not have enough seats in that class on the newer equipment, forcing them to downgrade you to a different class. I’m not certain how any given airline would handle that particular case, though.

Specifically in the case of Turkish Airlines, all I found on their website about it is just this FAQ entry:

What happens to my seat if Turkish Airlines has a schedule change or an equipment change ?

You will be informed about the change and a new seat selection (same or similar to the previous one) will be made.

Yes, you purchased transit by air, not a particular style of it. Actually, depending on whether it is an IANA governed flight or not, you may have purchased a specific weight and volume of transit space to utilize, and not actually the guarantee of personal transit. (Which is how airlines can refuse boarding to over-large passengers or require they purchase more than one seat.)

Only if your purchase agreement states that you are guaranteed a flight on a particular craft do you have a promise of that craft. This will usually only happen on charter/private flights.

It’s true that you can often see details of the aircraft on the website now, so you could have some expectations or think you have a contractual agreement to be flown on that aircraft. But in practice, airlines merely promise a best effort to bring you somewhere and can and do do a lot of things (switch airplanes, reschedule, cancel a flight outright, change the itinerary or even the means of transportation, etc.) that impact the conditions in which you will travel.

In this particular case, if you check Turkish airlines’ conditions of carriage, you will read that

Carrier undertakes to use its best efforts to carry the passenger and his or her baggage with reasonable dispatch and to adhere to published schedules in effect on the date of travel.

That’s really pretty vague, they don’t promise much beyond getting you to your destination (or, in fact, merely trying to get you to your destination). Even regulations on monetary compensations for significant delays or the right to be informed about the name of the airline operating the flight (not the actual aircraft or aircraft type but simply the name of the airline) are relatively recent so it does not seem they have to do more than that as far as the law is concerned.

Note that in the EU, there are strict rules about compensation for delays so it certainly seems rational for the airline to bring you as soon as possible to Madrid in a slightly less comfortable aircraft rather than wait for the other one to show up and possibly be liable to pay some compensation to all passengers.

Airlines have the right to change aircraft as they see fit. It is a quite common occurrence. Sometimes because of mechanical issues, sometimes because the passenger load would be better handled by a different aircraft, sometimes because they need to aircraft originally assigned somewhere else, etc etc.

While this time you got an older aircraft, it is just as likely that the change could be a newer aircraft. Just the luck of the draw.

And when these changes occur all bets are off on seat assignments, as the airline has the right to move people around as they see fit, no guarantees of retaining your originally chosen seat and no recourse or legal channels to follow up afterwards.