The F1Treasury may, F2... if under the circumstances of the case they think fit, accept any security in lieu of any security previously given to them, or of any part of such security, and that subject to such terms and conditions as they direct; so, however, that the substituted security shall be of the character which F3the Treasury might take if F4the local loan were originally granted at the time of such substitution, and that no change of security under this section shall extend the period for the repayment of F4the local loan.