When loss of coverage triggers the special enrollment period, the new plan is effective the first of the month following the month when you enroll. (verywell.com)
In that case, you'll have access to a special enrollment period during which you can pick a new plan, triggered by loss of coverage, which is a qualifying event. (verywell.com)
Of course, if there is a special circumstance, employers must make an exception if their status changes, either through a death, birth, or loss of coverage etc.. (echealthinsurance.com)