Is this possible? - Mailing list pgsql-sql

Hi,
I'm not to sure this is possible.  

I need to replace a primary key (pkid) with the value of a different field.
I have 
pkid = 200
attendid = 301

I need the pkid = 301

But there may or may not be a pkid that already exist that has the value of 
301.  The attendid is unique and the pkid data type is serial (has a 
sequence).

Thanks for the help.

-- 
John Fabiani