HOW ALTER PRIMARY KEY

If the data already exists, then this is a non-trivial task, especially if inheritance or Parent/Child is present, since this will lead to a change in the storage scheme of your data.

1. create a new class with the same structure, but with a new primary key;
2. move data from the old class into it using SQL (not the merge command);
3. delete data/indexes in the old class, then change the primary key in it;
4. move data from the new class to the old class, using the merge command;
5. delete the new class with the data;
6. rebuild the indexes if there are any.

Useful links: MERGEPersistent Objects and InterSystems IRIS SQLIntroduction to Persistent Objects

If you feel insecure with Caché/IRIS, it is better to ask WRC for help.