SL Green Realty is altering some ground-floor leases at the Graybar Building, at 420 Lexington Avenue between 43rd and 44th streets, to simplify the building’s ownership structure before it gets a new mortgage. Several groups have ownership stakes in the building, though it is managed and leased by SL Green. The company bought one ownership stake from W&M Properties early last month for $7.6 million, according to public documents. W&M’s Anthony Malkin said SL Green had approached W&M several times to sell portions of the building. SL Green’s existing mortgage on the building was originally due next year, but the company was able to refinance thanks to the W&M deal and several others.