If the order isn’t complete and the seller tried to change their gig, they would have to notify you in writing that you owe extra for the rights. Why would you accept and complete a gig based on that? That doesn’t make sense. You just tell the seller that the original terms need to stand and that the ToS says that the rights will be long to you as the buyer upon completion. If the seller tries to argue, you can ask for a cancellation and walk away. If the seller refuses the cancellation, you can make a Customer Support ticket. Support will be able to see the chronology and they will cancel the order for you if necessary. You are never under an obligation to accept delivery when the seller changes the terms after the order is placed. The buyer has the control. I’m really not sure what is complex here:
A. If the order isn’t complete and the seller tries to change the terms, then you don’t accept delivery. If the seller tried to changed the terms, Support will not allow it
B. If the order is complete and the terms said you didn’t have to pay extra for rights, you already own them.
Under either circumstance, you own the rights on a completed order if the work is original, and if the order isn’t complete, the power lies in your hands as a buyer.
Mario’s comment is about a different issue, which is about using a seller who tries to sell you something that never belonged to them. You can generally check work out in a reverse image search (or Copyscape for writing) If it’s not original work, don’t accept it. If it is, the rights are still yours. Fiverr doesn’t take responsibility beyond that for what independent contractors (sellers) do.
I think this has been twisted many times and my answers are still the same. Good luck!