All tutorials I found were how to fresh install worker MPM + PHP-FPM, since my wordpress blog's already up and running with prefork MPM, correct me if I'm wrong in the simulated installation process:

I'm on ubuntu and according to some tutorials, the following lines would do all the tricks:

apt-get install apache2-mpm-worker libapache2-mod-fastcgi php5-fpm php5-gd
a2enmod actions fastcgi alias

Then you setup configuration in /etc/apache2/conf.d/php5-fpm.conf:

<IfModule mod_fastcgi.c>
                AddHandler php5-fcgi .php
                Action php5-fcgi /php5-fcgi
                Alias /php5-fcgi /usr/lib/cgi-bin/php5-fcgi
                FastCgiExternalServer /usr/lib/cgi-bin/php5-fcgi -host 127.0.0.1:9000 -pass-header Authorization
</IfModule>

After all these, restart:

service apache2 restart && service php5-fpm restart

Question:

1) Would it cause any down time in the whole process for previously running sites with prefork MPM?

2) Do you have to change any already existent configuration files like php or mysql or apache2(would they take effect immediately after the switch without you doing anything)?

3) I've already have apc up and running, do you have to re-install/re-configure it after the switch?

4) How do you find out if apache2 is working in worker MPM mode as expected?

Thanks a lot!