upgrade target directory

I used the mapping file to direct the upgrade, via dbua, to be written to a parallel dir like this:

/db01/db/system01.dbf=/db01/db12102/system01.dbf

Not having much experience with this process, I was surprised the files were removed from the src directory.  It made me wonder if I could have just reused the original src directory and written to it as the target, like this:

/db01/db/system01.dbf=/db01/db/system01.dbf

Would this work?   Same for the fra, could I tell it to reuse the fra?  This makes me nervous since that is the location of my recovery data to be used on failure.