Non-canon
After re-reading Half-Blood Prince I've realised that Slughorn residing in Merrythought's old office can't be considered canon. After entering the Pensieve to view Slughorn's altered memory Harry immediately recognizes his surroundings as Slughorn's office. Therfore Slughorn used the same office during both his tenures as Potions Master. Jayden Matthews 08:55, April 20, 2011 (UTC)