Provided, however, that if the amount of the lien to which the county treasurer’s deed shall be made, as provided by Section 2 [73-13-20 N.M. Stat. Ann.] hereof, be not fully paid within six years from the date of such deed, the county shall, at the expiration of such period, be and immediately become reinvested by operation of law, with the title conveyed thereby: and this amendment shall apply to tax deeds heretofore issued if three years have not expired after the date of said deeds and prior to the time this act shall go into effect.