Is it permissible to move a deceased baby girl from her current grave (burial chamber) to another grave if one becomes available later, and is there a specific period within which such a transfer is permissible?

Burying above ground is permissible out of necessity. If there are no other cemeteries available, then there is no harm in burying there, and the burial has taken place correctly. It is not permissible to exhume the deceased from their grave except for a need, such as if the first burial place contains something that would harm the deceased.