aadel,
AFAIK, for this type of conversion, you will need text file (24414.2.$et) of your model. Required process should be as under.
1. Open text file of the model in Notepad.
2. In 'PROGRAM INFORMATION' portion, make following changes:
a. Change the PROGRAM option to "ETABS", INSTEAD OF "ETABS 2015"
b. Change the VERSION option to "9.7.4", INSTEAD OF "15.x.x"
3. Save the file as 'All files', with extension '.$et'
4. Import the newly created text file into ETABS 9.7.4.
5. Since ETABS 2015 contains a lot more new features (not available in older version 9.7.4), you will certainly see a number of error messages during this import, indicating the inability of program to import data regarding newer options and ignoring of the relevant commands. This is normal for imports of NEW version files, in Older versions of programs. Just keep on pressing 'OK' button on these messages, until import process has completed.
6. Save the imported file as ETABS 9.7.4 model (.edb) file.
Note: You must be aware that the model file, imported by ETABS 9.7.4, would however not be same as the original ETABS 2015 file. Rather it would be deficient in several respects, as may be clear from the error messages displayed during import in older version. Therefore, you will have to check the model in detail, to see which respects the model is OK, and in which it is not.
I don't know what an 'mdp' file is, as such I am unable to comment on export of such files in ETABS 2015.
Regards.