Is it possible to change from customapache to custombuild?
Yes. We've done it many times while updating older systems.
What are the pro and cons?
Pros:
More software updates controlled automatically (still only within single major version; for example, it won't
automatically update between PHP4 and PHP5 (though you can set it to do so). Updates much more than just Apache and other hosting software.
More flexibility. You can set major versions on many software packages.
Can be set to notify you if newer versions become available.
Can be set to update automatically when software is available.
If managed properly, along with proper management of your OS distribution's package manager, it almost completely eliminates the need for manual updating.
Cons:
None, really, in our experience, if carefully managed; see below.
What are possible problems?
During the conversion itself you should be careful that you've set up the configuration file to use the same major versions of software you're already running, to avoid any unintentional version changes.
Make sure your settings in the configuration file make sense. Ask questions here if there's anything you don't understand, specifically concerning things like the exim.conf file and the Dovecot configuration if you're not already using dovecot.
Note that automatic updates could cause some intermittent downtime to certain daemons; schedule them during times that won't be an issue.
Of course I don't guarantee I haven't left out any issues. If you don't feel comfortable doing the initial switchover yourself, or even continued updates automatically, you should contact us or some other server administrator on these forums which offers administration services. We and others guarantee our work but only when we're doing it; not when people are following our posts.
Jeff