http://www.google.com/apps/intl/en/business/index.html
With a 99.9% uptime SLA:
http://www.google.com/apps/intl/en/terms/premier_terms.html
and
Changing email addresses is a pain -- better to do it now, while you can slowly transition with email forwarding. Use filters to auto-label mail that is sent to the old address, ask senders to change your address, and switch over any mailing lists and accounts as automatic emails come in.
This would still leave you with the problem of your history of emails being stuck within Gmail.
Additionally, most spam filters take the sending host into account ("this host is known to have sent a million spam messages today - let's not bother talking to it"), but this information is lost (untrustworthy) if you forward via another mail server; it's likely that GMail has a better spam filter than your cPanel provider, so try to give it all the information you can.
There are some minor other issues - faster delivery, less wasted disk space on the cPanel provider's end, message sizes are limited only by GMail - but the above are the main ones.
Do note that you need GMail "for domains" instead of just a @gmail.com address. The "for domains" version is free, though (perhaps up to some upper limit - 50 mailboxes?).
(Meantime, keep your emails backed up.)
I also hope, a bit, that the paid version means Gmail/Apps may have some degree of independence/durability that is separate from the advertising revenue. I'm probably wrong, though, at least at this point.