You can't drop a Primary Index in Teradata.
Your initial assertion that its a primary key but not part of the PI cannot be correct. If you specify Primary Key in your Teradata DDL, TD will create it as a unique primary index, so your columns are indeed part of the PI.
If "Primary Key" was specified in your DDL, the only option you have is to create a new table with duplicate columns and the desired PI, insert from old to new, validate the data, drop old table and rename the new table to the old table.
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