according to this link http://social.msdn.microsoft.com/Forums/sqlserver/en-US/04d69ee6-d4f5-4f8f-a115-d89f7bcbc032/how-to-alter-column-to-identity11 "You can't alter the existing columns for identity." It provides a couple of possible solutions. Relevant commands are:
alter table YourTable drop column OldId
alter table YourTable add NewId int identity(1,1)