Achieving a highly available upgrade is a challenge for even the best IT shops . Everything about upgrades is ultimately a function of time. Upgrades, highly available or not, require a great deal of planning and testing before you actually implement them in your production environment. Remember to take into account any SLAs when you are planning your upgrades, as there is always a chance that you might not achieve them should the upgrade require you to be down longer than the time allotted in the SLA itself.
Doing your homework should pay off. If no one notices any functionality differences after the upgrade and downtime is minimal, this provides the best situation possible for everyone involved. If you do not do your homework, it could mean a great deal of both short-term and long- term pain. Upgrades of any sort should never be taken lightly. Mitigate your risks and, whenever possible, upgrade to completely new hardware so that your old environments provide the perfect fallback plan.