7zip - How to overwrite a existing file?

Question

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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 don't compress nothing and sends me this output:

System error:
File already exist.

I've tryed with the switch "-y", but 7zip is not doing any question...

So... Can't overwrite the file?

UPDATE

I've tried the switch "-aoa" but does nothing too...

http://docs.bugaco.com/7zip/MANUAL/switches/overwrite.htm

UPDATE 2

I need a solution using the parameters of 7 zip, not deleting the files first.