The seven-story downtown Worcester office building long known for its anchor tenant, the insurer Unum, will undergo new branding and leasing now that Unum has elected to have its area employees indefinitely work remotely. Franklin Realty Advisors, which owns and operates the Mercantile Center office and retail complex across Mercantile Street, said Wednesday it has been hired to manage, develop and oversee leasing for the building, which according to city records spans 179,000 square feet. The building will be rebranded One Mercantile after its street address. Unum, whose name adorns the top of the building, said last July it would keep employees working remotely indefinitely, after first having switched to people working from home once the coronavirus pandemic hit. The Tennessee-based company had roughly 400 local employees at the time, and according to Franklin Realty Advisors, the insurer occupied 125,000 square feet of space.