Nor can the caster “drop” the animal form although they can use an action to transform a target into a new beast. It is a concentration spell so the caster can choose to end it for everyone at any time.
There are no secret rules. If it doesn’t say it, it doesn’t do it.
See Can you make an unwilling creature willing? In other words, what defines “willing”?
This is a DM call as to whether a “willing creature” must be willing only at the time the spell is cast or must remain willing throughout the duration to remain a valid target.
Unlike most other circumstances where a target becomes ineligible, deciding to be willing or not has no cost.
It also seems to treat “willing” as some sort of ‘active’ state that you must consciously remain willing or the spell fails – what if, momentarily, you doubt the sense of being a lobster? Does that momentary doubt make you unwilling and drop you back into your original form?
Similarly, if you are rendered unconscious or go to sleep does the spell end? The designer suggests an unconscious creature is not willing.
As a DM, I would not want to deal with all that and would rule that if you are willing to have the spell cast on you, it doesn’t matter if you change your mind later.