Look up ALTER TABLE in books online (web version: http://msdn.microsoft.com/en-us/library/ms190273%28SQL.90%29.aspx)
ALTER TABLE ... DROP CONSTRAINT ... to remove the pk
ALTER TABLE ... ADD ... to add a new column
ALTER TABLE ... DROP ... to drop the old column
ALTER TABLE ... ADD CONSTRAINT ... to replace the pk constraint.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability