Employers ask about availability as a courtesy, not as a matter of law, and they are not legally bound to abide what you give them as a matter of employment law.
As a matter of contract law, if there is a *written* policy that they have heretofore always adhered to in requiring your written consent before changing your schedule, it is *possible* that it might be contractual. But only an Oklahoma attorney who has read the exact wording of the policy could say for certain.
I have to agree with the other two posters that it is likely that this is not a contract and that your options are to either work the revised schedule or find other employment.

