If you are working with a AXDBLib document you don't have the option to change file types .. if you are opening the drawings though, you can save them as a different file type by specifying the filetype constant.
If you want to change the document without opening it, then you cannot change the filetype .. however, since the TrueConvert utility is now a free download, you may be able to utilize some of the items in that program to achieve the version changing you need without opening the drawings