According to USCIS guidance memos, one can apply for the change of employer as long as the authorized period of stay has not expired. (Although we have seen some CIS adjudicators refuse to give an extension of stay if the person is no longer with the original employer at the time of filing, in this case we are not asking for the extension, only the change. We can always request the extension when the change has been granted and your current authorized stay is about to expire.