Compression of .root files

Dear rooters,
I have a question about compressing .root files:
Is it possible to change compression level after writing the file? I have a lot of .root files which need a lot of disk space and unfortunately gzip and tar fail to compress these files. So I wonder if there is a root function which reads the .root files, changes the compression value and re-writes the files, or if I have to do this by myself!
Thanks for your help!

No we have no tools to do this. You should rewrite your file with a different compression level.