Dmitry is correct: do not try to upgrade on your production server. You may end up with a mess. Create a new server, move routines/globals/classes to the new server and test extensively.
Difficulty in upgrading Cache depends completely on your application. If you use exotic features like $ZU, calls to the OS, third party software, discontinued functions, it will be harder to upgrade.
Tom Fitzgibbon | Multidata | 347-464-8531