You might ask 'why' I am doing this, but that's not the point. To update a field in a table in mysql, from the same table (which MySQL disallows):
set myvalue = myvalue+(select myvalue FROM (select myvalue from mytable where year='2008') b )
year = 2009
Basically, you have to trick mysql into selecting the subquery from a temporary table; as done above.
Kudos to Peter Geer for the idea for this.