Change Required Hours per Week for Salary Employees

I have worked for an Ohio company in a salary position for the past five years. I do not have an employment contract with this employer.
I was recently informed that the hours I was required to work during a week had been changed to 45 hours. When I took the position I was not directly informed as the number of hours per week and to the best of my knowledge there is no written policy as to the number of hours required. At no time during my employment have I been docked pay or reprimanded for working only 40 hours per week.
We were informed that if we could not work the 45 hours that an hourly position would be made available at a lower wage.
My question: Can an employer increase the number of hours required per week without a corresponding increase in salary?