Converting Non-Unicode to Unicode

What is the best way to convert a large 24/7 database to Unicode? We have
various varchar and text fields that need to become nvarchar and ntext
respectively. I know I can alter the tables to do the varchars, but that
would have to be offline (looks like like about 2 hours per column on my
50 million record table). Text fields can't be converted with an alter so
they have to be copied to a new column. I was just wondering if anybody
has experience converting these.In article <38E5F8AA-A720-4E27-A01F-9BB364263191@.microsoft.com>,
satyaskj@.yahoo.co.uk said...
> Why not design another table with new datatypes and use DTS or BULK insert to import data from the old table.
> Once data is propogated delete the old table and rename new to old to handle the application.
Because I need to do it as on-line as possible. If I do a copy I will
have to shut down the site before I begin and I know it will take quite a
while.