How can I overwrite an existing file in 7zip?

I'm trying this:

7za.exe a "C:\Disc.7z" "C:\test" -v20000000b -m0=BCJ -bd

But if I already have created the file C:\Disc.7z then 7zip doesn't compress anything and sends me this output:

System error:
File already exist.

I've tried with the switch -y, but 7zip does not do anything...

How can I overwrite the file?

UPDATE

I've tried the switch -aoa but it does not work either...

Your problem is that you're using -v to create a multi-volume archive. People have been requesting the author for the last 5 years to allow the program to modify multi-volume archives, but he has no plans to implement the feature any time soon. (It's open source, wish some enterprising programmer would do it already!)

Try using 7za.exe u instead of 7za.exe a. The first one is specifically used to update an archive that already exists.