You are not bound to keep your investment in the government bonds when it matures, however, this could affect your investor visa. Indeed, it is justified by a commitment with the Italian government so the withdrawal makes you not covered with general rules. Therefore visa will not be revoked until it expires, but with no investment made it will be very difficult to renew it and you will be able to keep the visa (obtained if you have met the initial 2-year retaining requirement for the government bond investment) only for the time been already granted.
The resident permit for investors lasts two years from arrival in Italy. Foreign citizens that hold all the requirements and have maintained the original investment can apply for a 3-year renewal.
The investor visa and resident permit will be revoked if the investment is withdrawn or abandoned within the two years after issuance of the visa. In fact, the investor visa holder shall maintain his/her investment for the entire validity period of the resident permit, failing which the visa and the permit will be revoked and cannot be renewed. The original investment shall be maintained and the holder shall not change the destination of the investment under any circumstances, otherwise, he/she will lose the visa and the residence permit.