Well, the first problem is that this question should really be on the "Data Translation" forum and not the "CAD Administration" forum. That point really is not that important in this case, however, because Solidworks is a CAD program that is not set up to save to older versions. Once you edit a file in a newer version of the program, you basically can not go backwards. Sorry, but that is just the nature of the beast.
There is a workaround, but it is not guaranteed to work every time. When you edit the file in the newer version, save it as an IGES or STEP file. Then import it into the older version and have the older version attempt to 'recognize' the features and recreate the geometry as if it were importing the model from an entirely different CAD system. It might work, it might not.
I wish I could offer you more help, but that is the best advice I can give you.
We currently utilize a solideworks program called PDMWorks. It is a vault which you are able to check in and check out files, most any type of files. Each time a file is checked in, it addes a "revision" to the file and does not overwrite the file. Thus, all "older versions" of the files are kept. These older versions can be from earlier versions of solidworks as well. I currently have some 2004 files in the vault and I'm using 2007.