[Koha] MySQL to update dateaccessioned

I am trying to update a bunch of various dates to 31 Dec. The following appears to be syntactically OK, finds all 1302 to be changed, but does not change them to 31 Dec. mysql> UPDATE items -> SET items.dateaccessioned=REPLACE(items.dateaccessioned,'%','2011-12-31') -> WHERE blah blah blah; Query OK, 0 rows affected (0.04 sec) Rows matched: 1302 Changed: 0 Warnings: 0 Help really appreciated ... Many thanks - Paul