Investment u/s 54ec (nhai bonds)

An assessee made an investment of Rs 40 Lakhs U/s 54 EC (in NHAI Bonds) to claim an exemption from Capital Gain on 20th Dec. 2014. Further the assessee died on 28th Feb 2015. Can the investment made in bonds be prematured/redeemed by the ledal heir's before the expiry of 3 years ? Further, in case the bonds are prematured/redeemed, whether the bond amount withdrawn will be taxable in the hands of the legal Heir's ?