Can my employer require me to change my PRN status to part time if I work more than 24 hours a week?

I'm a PRN physical therapist at a nationwide workman's comp company, a year or so ago, they tried to make PRN staff take a full time or part time job status, making less per hour, if working more than 24hours a week claiming it was a new rule when the company was bought out but I found out it wasn't happening in other states. Now, I'm being told that they are trying to have PRNs that work more than 24 hours per week to switch to part time again without the pay change. I don't always work more than 24/wk, sometimes it is more, sometimes less. First, can they make us change? Second, why would they want us to have a title change? Third, are there certain questions I should ask them?