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In almost all cases, you can, depending on the advanced features you want to add. Obviously, if your dream of UC includes video phones, you'll have to upgrade all your non-video phones, which probably means all of them.
But if your idea of UC means Find Me – Follow Me, IM, faxes, voice mails, and emails routed into the same inbox, your existing phones should work fine. Many times your phone controller can be upgraded, or tied into some UC features running on a computer, so you can maintain the bulk of your existing equipment. Beware of UC vendors who say "rip and replace" is your only option.
That said, most small companies change their phone systems when they move or when their equipment lease runs out. In those cases, upgrading your entire system may be cost effective in order to install equipment able to handle your UC features today and those you'll add next year.