Unfortunately, the L2 is a dependent based visa and status, which means the person is only eligible for this status if the principal has L1 status. So once you change your status on October 1, you no longer are on L1 status, and therefore, your wife no longer is eligible for L2 dependent status. It doesn't matter that her I-94 has a validity date of 2015. That date and validity assumes that you are still in L1 status as well. So once you are no longer on L1, she has no basis to be on L2. It simply is not possible in any way.
So to answer your question directly, no, she cannot remain on L2 status after October 1, when you change your status. She either has to apply for a change of status or she must leave the US.
Again, as far as the validity dates on the I-94, these reflect the expiration date of the I-94, ASSUMING that she remains eligible for the status. In this case, she would not continue to be eligible for L2 status once you go to H1B in October.
The decision on Premium Processing cases should come within 15 business days, if there are no delays like an RFE issued during the Premium Processing of the petition. There is no telling when it will change online. Sometimes the status update reflects the latest happenings on a case, sometimes it does not. And sometimes, a person will get a result in the mail before there is a change in the online status.
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