Redecorating the Oval Office has not always been undertaken immediately upon the transfer of power. Certainly personal effects such as family photos are removed and replaced, but furniture, carpets, drapes and artwork sometimes stay the same from administration to administration.

According to the White House Museum, Johnson did retain some of Kennedy's decor. When he first moved into the Oval Office in November 1963, Johnson kept Kennedy's red carpet and pale curtains in place. He did, however, replace Kennedy's desk with one Johnson had built for himself by the Senate cabinet shop. By 1966, Johnson had also replaced Kennedy's carpet with a pale blue one. Johnson retained Kennedy's curtains throughout his term in office. Nixon, too, retained Kennedy's pale curtains and Johnson's blue rug for a time. He was inaugurated in January 1969, and there are photos of him in the office as late as July 1969 with Johnson's decor still in place. Carter never redecorated, he kept Ford's final decor in place throughout his presidency.

Johnson moved into the Oval Office fairly quickly, angering Kennedy's staff, but did not move into the White House residence right away. I wrote about this here before; I apologize for the brevity but am on mobile.