Some 275 Harvard University employees will be offered voluntary buyouts in the school’s first concrete move toward modernizing its decentralized library system, university officials said yesterday. Workers there have worried about involuntary layoffs, which they were told to expect during a contentious set of internal meetings in January that led to protests. But the packages offered yesterday are “totally voluntary,’’ said a letter from Marilyn Hausammann, the university’s vice president for human resources. The affected employees are largely 55 and over, with 10 or more years’ experience at Harvard. The buyout packages will offer a payment equal to six months’ pay plus two weeks of pay for each year of service in excess of 10 years, up to the equivalent of one year’s base pay.