kesab,
As long as the principal applicant is on a "continously valid" H-1, the dependant can get the status converted from AOS to H4 with a simple "change of status" application. The H-4 is is granted for the same time period that the principal applicant's H1 I-797 is valid till. The key is that the spouse cannot be working or should not have used the EAD. The change of status application will specifically ask for that information.
The USCIS may issue an RFE asking for clarification (it happened in my case), but it can be easily responded to by the lawyer.
In my case USCIS did change status of my wife to from Advance Parole (DT) to H-4 with an expiry date that matched my I-797 and issued a new I-94 with the same number as the Advance Parole I-94, but with the status changed to H-4 and date of expiry that matched my I-797.
Hope that helps --