Licenses so granted shall be graded into two classes: One of which shall entitle the licensee to have charge of or to operate stationary steam boilers and steam engines only; the other of which shall entitle the licensee to have charge of or to operate portable steam boilers and steam engines only; such licenses shall not be transferred from one grade to the other without a re-examination, said re-examination to be conducted without cost to the licensee.
53 P.S. § 25313