Can heirs be removed from a trust by a codicil?

By the language you use in your question I will assume you are referring to a testamentary trust.

The answer is yes. If you have created a testamentary trust in your will you may make any changes during your life by using the form of a codicil. However, make certain the codicil is properly drafted to meet legal requirements and clearly written in order to make your changes perfectly clear. You must state the section you wish to revoke and then clearly state it's replacement if there is one.