Thank you for the information and your question. There is nothing adverse under the law about an employer changing an employees wage payment from salary to hourly, so yes, the employer could keep you on an hourly basis after you come off of FMLA. What they can't do, unless there is a reason other than your use of FMLA, is to change your actual job. The way in which you are paid for it is not an issue under the law (assuming you are not being paid less for the hours you actually work than you were before.)
As an aside, an employer always has the right to pay all of their employees on an hourly basis. They don't, however, have the reverse right. In other words, only certain types of employees may be paid on a salary basis with no right to overtime. Sounds a bit confusing I am sure, but the reason that all employees can be paid by the hour is that they actually have more rights under wage laws that way, in particular the right to overtime, than they did when paid by salary.
This answer, of course, assumes that there is no written term contract that the employer is violating by paying in this manner. Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the box above, I will receive credit for assisting you today. Thank you