[Koha] MySQL to update dateaccessioned

[Paul]

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