Sections 58.1-803, 58.1-807, and 58.1-808 are not to be construed as requiring the payment of any tax for the recordation of any deed of trust, deed of subordination, mortgage, contract, agreement, modification, addendum, or other writing that is supplemental to any deed of trust, mortgage, contract, agreement, modification, addendum, or other writing theretofore admitted to record upon which the tax herein imposed has been paid, or which is exempt from the tax herein imposed by reason of subsection C of § 58.1-804, when the sole purpose and effect of the supplemental instrument is to wrap around a prior instrument, to convey property, in addition to or in substitution, in whole or in part, of the property conveyed in a prior instrument, to secure or to better secure the payment of the amount contracted for in a prior instrument, to alter the priority of a prior instrument, or to modify the terms, conditions, parties, or provisions of a prior instrument, other than to increase the amount of the principal obligation secured thereby. The assumption of a deed of trust shall not be separately taxable under § 58.1-801, 58.1-803 or 58.1-807, whether such assumption is by a separate instrument or included in the deed of conveyance.