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You should upgrade before or after the server change...
Either :
upgrade to 5.0.1, do a full backup, install new hardware, install Zimbra 5.0.1 on new hardware, restore full backup
or :
do a full backup (4.5.x), install new hardware, install Zimbra 4.5.x (the very same version than old hardware), restore previous backup, upgrade to 5.0.1, do a full backup