>1. Change an index from candidate to principal;

Whats principal mean?

Anyways, I would DELETE TAG and then add a new one with INDEX.

>2. Add a new index (of any type);

INDEX.

Somethings you can't do with INDEX, like create a primary key. Why? Cause that infos in the DBC, not the CDX or DBF. You can use ALTER TABLE to add primary keys, ect.