My boyfriend and I have lived together for 20 yrs, up until his children took guardianship of him 1yr. Ago, because of his diminished capacity with dementia. I have always been in his will Can they change his will?

No. They can't.

If he changes his will, and after his death they submit it in probate, the question will be whether he had sufficient mental capacity to make a will at the time he made the change. If a legal proceeding to establish guardianship has occurred, this issue appears to have already been decided.