Subject to subsection (b) of this section, any change or modification to a mortgage or deed of trust or to an obligation secured by a mortgage or deed of trust does not extinguish the existing lien of the mortgage or deed of trust or otherwise adversely affect the existing lien priority of the mortgage or deed of trust. If the change or modification to a mortgage or deed of trust or to an obligation secured by a mortgage or deed of trust increases the principal sum secured by the mortgage or deed of trust above the amount appearing on the face of the mortgage or deed of trust and expressed to be secured by it: The existing lien priority of the original mortgage or deed of trust shall continue as to the principal sum secured by the mortgage or deed of trust immediately preceding the change or modification; and The lien priority for the increase in the principal sum shall date from the date of the changed or modified mortgage or deed of trust.