[exim] performance tuning - split spool directory

Hi Guys,

I'm doing some performance tuning for on one of our pretty heavily
loaded servers, i'm wanting to enable the split_spool_directory
directive. Do I need to ensure the current queue/spool is completely
flushed before enabling this? Or, if I switch this option on and
restart, will the existing queue continue to be processed with new
incoming messages being filed into the subdirectories?

I'm trying to find the best window of opportunity to do this but
can't afford much downtime on this box.

Cheers,
Marcus