That's up to the Gate Agent. Flight attendants are not permitted to upgrade paying passengers.

You'll be in jail for false imprisonment if you try that. No crime against you has been committed. No crime against DL has occurred yet either. Unlike a law enforcement officer, you don't get immunity if you detain someone and it was incorrect. The only exception to this rule is for merchants and their employees who do get immunity. Even then it is very specific circumstances that they get it.

You likely have a civil case to sue DL, but no criminal case against the "thief". You don't own the seat, nor has DL leased you the seat. DL could file charges for trespassing if OP refuses to get off plane after being asked. (If they refused to move) If passenger moves back to C+ inflight then they could also possibly file theft of service charges.

I have a feeling that the FA already decided that they didn't like OP before the whole seat poacher thing. Why? Looking at DL1943 on FlightAware, this flight has been operated by a 737-900ER for as long as the history on FlightAware shows. Row 10 on this aircraft has a closet on the DEF side. If there was no room for OP's bag, most FAs would offer to stow it in the closet, or take down one of their crew bags and stow it in the closet to make room for the passenger's bag.

preferably citing also safety concerns about passengers from row 33 moving to row 10 and crew not correcting it when made aware.
Very sloppy work on the part of the cabin crew and demonstrates willingness to cut corners in order to get out faster.

How much do you want to wager that those bins were full because the squatters’ bags were in there?

The pax had been assigned to 10C, but taken by someone else not supposed to be seated in 10C. How can someone be refusing to take their seat if their assigned seat had already been taken by someone else (improperly)? Stand up for your rights. If you have to be dragged off the flight, so be it. Call out the wrongs.

Squatters were ticketed in row 33, and decided to move themselves to row 10. FAs knew those squatters were in the wrong and still did nothing.

What safety/health reasons are you referring to in this scenario? Is there someone wrong with the ticketed seats where they are?

All fair points about this being a poor showing by DL attendants. I am willing to assume the 10C pax has agreed to the swap so that they are not seen as the reason for the delay by other pax.
Given his (eventual?) cooperation that should've made their jobs easier, FAs should've at least tried to settle the matter with the inconvenienced pax there and then

RE: Squatters

Ah, not a lawyer, but I'm assuming in the post Dao era, DL can't really force the squatters to move if they refuse? I mean, you can use force, right? Can't call the cops, right? Offer a bribe...er...compensation to return to their assigned seats?

this last bit wouldn’t wash at all. The lady has two seats together. That’s all she needed. If the original 10C pax simply refused to fly in another seat they would have had to offload the bags and further delayed the flight. No need to get drug off the plane.

(Bold emphasis mine)
Not saying the seat squatters weren't wrong (they were). Not saying the FA wasn't wrong (the FA was wrong). Not saying I wouldn't be furious if this happened to me (I would be). But the reality is it's always so easy to be a "tough guy" on the internet where real world consequences don't apply. Regardless of the situation, how would most employers view you getting dragged off a plane and possibly arrested? And even if you got off the plane willingly (not by force) but chose to deplane over the seat and take another flight, if you were on a business trip and you missed the meeting or event you were going to and you told your employer "I chose to get off the plane rather than take the other seat based on principle," how would they react? In the real world, people don't get to make such decisions in a vacuum free of outside consequences and impacts.

He only agreed after repeatedly being told not to make the flight later than what it already was and he kept complaining that he needed the aisle because of a bad foot. But in all the time they were telling him not to make the flight later than what it was the squatters could have moved to 33 which I assume were still empty. Not sure haven't made the trek that far back of the plane in a long time.

Moved Seat
 

Why weren't the people moved after they were in the air? That would have resolved all the issues. I would have made a stink as well. Need to be reported to FAA. Staff was just plain wrong on this one.

The FAA and DOT aren't going to get involved or probably care. There was nothing "illegal" and, right, wrong, or indifferent, the airline can *always* fall back on "seat assignments are not guaranteed and passengers may be moved to another set for operational reasons or to accommodate other passengers with special needs." The issue is between the person who had their seat swapped and DL and it can be resolved there. What should happen is: 1) DL provides the person who was moved appropriate compensation, even if he was accommodated in the same "cabin" (C+) and 2) DL needs to let the FA who took the easiest and most painless route for them was extremely inappropriate and outline the action the FA should take in the future (move the seat squatters). However, DL also has to tread carefully in how they approach the FA because all of this is technically hearsay as far as DL can tell from its own records.

Dr. Dao was seated in his assigned seat, a different issue from what is being discussed here.

Once in the air how do DL FAs get the poachers to move if they refuse to "budge"? An incident in the air is to be avoided at all cost.