Can my boss switch me mid year from W2 to 1099 without telling me?

She can make the change at will IN THEORY (see below), but it is only effective from when she tells you forward (not retroactively): an employer can decide whether to employ someone as an employee (W-2) or indepenent contrator (1099), but to have someone work (and pay them) as a contractor, the worker *must* meet the criteria to be an independent contractor...that's why we say they can do this "in theory," since you'd have to be validly considered a contractor to be paid on a 1099 basis.
The law doesn't care what an employer wants to call the working relationship or worker; it cares about the facts. To be an independent contractor, the worker must, as the term implies, have a degree of independence. If the employer sets your hours; can determine where you must do the work (e.g. you can't decide to work at home or a Starbucks), tells you not just what they want you to do but tells you how to do the job (manages how you do your job), etc., then you are an employee, not a contractor--you do not have the requisite independence to be an independent contractor. So if you are working as an employee does, you have to be treated and paid as an employee, not an independent contractor.