You'll probably need an integer column that's an identity to make this work easily. Lets say the column 'ID' is an integer that starts at 1, and increments by 1.
Then you could do something like this.
DECLARE MAXID INT
DECLARE CHANGEID VARCHAR(20)
SET MAXID = SELECT MAX(ID) FROM CHANGE --Gets the highest ID value
SET CHANGEID = 'HCM-000' + CAST(MAXID + 1 AS VARCHAR) --Builds the Change ID value
After that, you can use your new CHANGEID in an insert statement
However, if you're just intersted in changing the prefix of a task ID (i.e CM-0x -> HCM-0x), this is all controlled on the Reporting tab of the project properties (click on parent node of project, hit reporting tab). You can manipulate the prexix and padding of the task ID here.