I've created a subtype field in my attribute table, but now I want to change the codes.
How do I do that?
ArcCatalog doesn't enable editing my subtype.
You need to obtain schema lock, i.e you must be the only one connected to the database. Then you change the values in the domain connected to your subtype field.
At least before version 10 using SQL server you couldn't change a value within a domain, only add and delete at the end. To change a value you have to delete from the bottom up to the value you want to change, save, and then add back the new values. If using another database or version 10's new domain storage format it might work anyway, I haven't tested.