Hibernate hbm2ddl.auto=update doesn't update column definitions in MySQL

I'm trying to update existing table with hbm2ddl.auto = update.

There is several columns in several tables where database column definitions changes from declaration in entities. Like

@Column(name="mycolumn", nullable=false, length=10)
private Long mycolumn;

and

'mycolumn' bigint(20) not null default 0

in MySQL.

Why hbm2ddl doesn't update such things? And is it possible to force such update? I want to say hbm2ddl to remove default value of column and change length of type.