The new owner of an Oak Lawn office building — Abby Development — plans to begin a multimillion-dollar upgrade to the aging building, which could re-position the property, which sits near the popular Uptown neighborhood, to attract key tenants. The upgrades to the four-story, 48,000-square-foot building at 3333 Welborn St. are slated for completion by the end of the year, which will bring nearly 30,000 square feet of available upgraded space to the market. "We have spent the last several months…