A private letter ruling issued under this subtitle may be relied on solely and prospectively by the person for whom the private letter ruling is requested unless there is an intervening statutory or regulatory change or the private letter ruling is revoked by the Comptroller. A private letter ruling is binding on the Comptroller for a period of 7 years unless: there has been a misstatement or omission of material facts in the request; the actual facts are determined to be materially different from the facts on which the private letter ruling was based; there has been a change in law or a final decision in a contested case that the Comptroller determines affects the validity of the private letter ruling; or the Comptroller has modified or revoked the private letter ruling. An unfavorable private letter ruling does not bind the person for whom the private letter ruling was requested. The modification or revocation of a private letter ruling by the Comptroller may not be applied retroactively to taxable periods or taxable years before the effective date of the modification or revocation.