There is no conversion of accdr files to accdb. You can rename the extension. One of the first tests I did last night was to see if I could break an accdb file by using a computer that did not have access installed and renamed the extension. It did not break. I did the same on a computer with the wrong version of access. It did not break. I have been able to, in the past, break files by accessing the same accdb file from different computers using different versions of access. That is likely what happened to your file.
I looked into switches and I can not find any that mention renaming the extension. I could do it VIA MSDOS but I do not see the reasoning plus, I would have to dig to find my notes. I could rename the extension using VB Script, which will do the same; use Windows Shell to rename the file. I am not convinced this approach is different from using the GUI and clicking the file to rename it.
Assuming W7 64bit and Access 2007 32bit, here is a switch that will run a repair but it will probably not work because of the fact the file will not open.
Code:
C:\Program Files (x86)\Microsoft Office\Office12\MSACCESS.EXE "C:\FolderName\FileName.accdb" /repair
The only other thing I can suggest is zip down a copy of your file and email it to me to see if I can help.