[Dovecot] Zlib plugin: changing compression save level

Hi,

I have running dovecot 2.1.15 with zlib plugin enabled and "zlib_save_level = 6" since about 2 years now without any problems.

What happens if I now change the zlib_save_level to 9? Should that work without any problems, or become the current saved "*.m" files incompatible or unreadable?

Thanks Urban