A company may make changes in coverage going forward, from when they announce the change. They may not retroactively or after the fact (i.e. after a cost or bill is incurred, or medical services provided) change coverage to an employee's detriment; doing so is effectively a violation or breach of contract, since it is to not provide the compensation which you had performed the work/employment for.However, a  company does not have to go out of its way to make sure an employee knows  of a change--therefore, while if there was no notice of the shift, the company would seem to have to reimburse you for this cost, if there was any memo, announcement, email, etc., previously, then even if you personally forgot it, didn't pay attention, etc., the company provided the requisite notice.Note, however, that even if the company should pay for this dental work, if they refuse to do so, you'd have to sue your employer for compensation, which has its own obvious costs.