WASHINGTON — The Army is planning a sweeping transformation that will merge or close headquarters, dump outdated vehicles and aircraft, slash as many as 1,000 headquarters staff in the Pentagon and shift personnel to units in the field, according to a new memo and U.S. officials familiar with the changes. In a memo released Thursday, Defense Secretary Pete Hegseth ordered the transformation to “build a leaner, more lethal force.” Discussions about the changes have been going on for weeks,