Let me guess, you use the SSMS GUI table designer to make the change?
The table designer creates a new table with the new schema, moves the data across and drops the old table. Hence the identity could reset, it's a new table.
Don't use the table designer, write your table schema changes as ALTER TABLE statements and you won't have this problem
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