How to limit power of trust protector

Yes, your relative can change the terms of an existing Will.  The amendment is called a codicil.  The codicil, which can be handwritten or typed, should state that your relative reaffirms the existing Will except for whatever provisions he or she wants to change, and then specificallly state what provisions should be deleted and add the new language.  The codicil should be signed by your relative and two witnesses.  A separate sworn affidavit of the witnesses, signed by a notary, is useful although not necessary.   Your relative could give the beneficiary, a co-trustee or some third person the authority to remove a trustee.  It is customary in a special needs trust to give the trustee broad authority, but also the requirement that they act in the best interests of the beneficiary.  There also a pooled special needs trust that your relative may want to consider - these are administered usually by nonprofits and can be less expensive.