To answer your question, I need to know who is needing a new guardian for a trust fund. If the trust can be amended by the trustor (person who creates it), all that needs to be done is simply have an attorney that does Wills and trusts work prepare an amendment to the trust setting forth the name of the new guardian to be signed by the trustor.If the trustor has passed, then you need to carefully read the trust agreement in that the document might set forth a means to establish an alternative guardian for the trust. If there is no mechanism for this, then a petition needs to be filed in the county and state where the trust was signed.