Can an employer change your medical insurance provider?

Yes, it is standard practice for employers to search out the best value for the dollar, and switch at the end of the contract year if they find someone better.

That said, the law governing health insurance has changed over the last number of years, mostly in a way to broaden the availability of coverage for the employer. In years past, what often happened was that when the insurer changed, there was a period of time when a pre-existing condition was not covered under the new policy. A pre-existing condion is one that existed before the then-current policy went into force.

New laws made it impermissible to exclude those conditions if the insured had continuous health coverage for a period of time before the new policy went into force. The amount of time that the old policy had to be in force may vary (often 90 days), so it is important to check. The point is, that you should never let the health insurance lapse or otherwise go out of force, so that there is a gap in continuous coverage into which the pre-existing condition may fall.