Do you mean public to private or the other way around?
If you mean that you enter something in the personal calendar and want it to
go to a public folder then you can do that exchange server based without
client install using the exchange add-on on
http://www.exchangeGroupCalendar.com

Dear Tracey,

The questions is, if this kind of function will be practical.

If you would have 50 users, and everybody will synchronize his personal
calendar  with a public calendar, then you will have also all dates of the
50 persons in one calendar. What a disaster.

If you would have 50 public "personal" calendar it is the same disaster.

For a few persons it could be a good solution but not for hundreds of people
with one Exchange (Public Folders).

But if you share your calendar with other on exchange, you don?t need to
synchronize it,, you can share it with Exchange.
To synchronize (for example 50 calendars) it will take a lot of network
traffik. Share your calendar and don?t synchronize it (my meaning).

--
Oliver Vukovics
Share your Outlook without Exchange: Public ShareFolder
Notebook Synchronisation for Outlook: Public SyncTool
New Domain: www.publicshareware.com