Is it legal for an employer to switch your from an employee to an independent contractor?

It is generally legal, since all employment is employment at will (unless you have a written contract to the contrary); this means you employment could be terminated and then if they re-hire you, it would only be as an independent contractor if that's what they want. But there are several limitations on this:

1) There must be notice to you; they can't do this without telling you.

2) It is only prospective, not retroactive: it is ony effective from the time you are provided notice forward (so if never provided notice, it would *not* be effective).

3) It only is effective  or legal if you meet the criteria to considered an independent contract, such as: you, not your employer, controls how you do the job (they can tell you what they want done; you decide how to do it); you control your hours worked, not them; you provide you own equipment, supplies, etc.; and not hard and fast, but typically, you market your services actively and have more than one "employer" at a time or in relatively quick succession. If that's not the case and if in all ways you still functioned and were supervised as an employee, then you would still have been an employee.

So depending on what happened, how you work, the notice you were provided, etc., this may not have been an effective or legal change. In particular, if you work on site, on their hours, 5 days a week, 8 hours per day, this is your only employer, and they tell you how to do your job, you are almost certainly legally an employee, not a contractor, not matter what they want to call you; and if an employee, they had to withhold taxes for you, provide benefits if their employees get benefits, pay overtime as applicable, etc.