I would imagine your professor is trying to put some pressure on you. Maybe (likely!) you did not discuss the authorship details in enough detail in advance. Unilaterally changing the authorship is unethical, however, walking away from a group project in the worst possible moment without facilitating the transition in any way also leaves a bad taste. If the paper was rejected and due to resubmission, the renegotiation would be more straightforward.
A case could be made for the professor's perspective, if the revisions are a substantial amount of work: they would require someone else to do what you did almost from scratch, relying on the description in the first version on the manuscript alone. This could be similar to abandoning the paper altogether and starting it anew, where your contributions to the lab would be recognized but the claim to the first authorship would not be as rock solid. Walk a mile in their shoes, this scenario is fairly bad for both sides. You are not obliged to keep working for them, but they are not obliged to see this paper to completion, either, and hardlining the "not my problem" approach well may tarnish your professional reputation. Especially if you knew in advance you would be taking up that job demanding pretty much all of your time and energy and did nothing to mitigate the issues you are facing now (admittedly, this is not exclusively on you though, the professor should have raise these concerns before).
Having said that, I would advise you to focus on what you are and are not willing to do for this work. If you could make yourself available to someone taking this project over and help them with some questions, it would be reasonable to demand no changes in authorship. Otherwise, I would probably give it up if I were you: even if you have played the leading role in the paper, not cooperating with the rest of the team just enough to see the paper to completion is a bad move. As is usual in negotiations, a good strategy is offering the other side choices instead of shutting down their propositions.
Find something that works for all the parties involved. You can take a hard stance on authorship, but it is then on you to describe what will your responsibilities be and how are you planning to get this paper published. You could ask for an extension of resubmission deadlines, offer consultations or try to find time to work on it yourself, anything really as long as you do not just walk off. See the other side of the story, too.
The rest is as easy as stating "sorry, I do not think the change of authorship is appropriate".