Can employer change pay status from hourly to salary without notice then refuse to pay overtime?

Yes they can but they must also give you a reasonable amount of time as notice which is normally a week or a month depending on the company's policy regarding payment changes. As to refusing to pay overtime, salary pay is payment for a job done to completion so if it takes you five hours to do and you get paid for eight then basically get paid 3 hours sat in the canteen drinking tea, but if the job takes you 10 hours, then you only get paid for the 8 hours.