To avoid unnecessary work the mysql_alter_table function takes the
list of table fields and applies all changes to it (drops/moves/renames/etc).
Then this function compares the new list and the old one. If the changes
require only .frm to be modified then the actual data isn't copied. To detect
changes all columns attributes but names are compared. When a column has been
moved and has replaced another column with the same attributes except name
the mysql_alter_table function wrongly decides that two fields has been just
renamed. As a result the data from the moved column and from all columns
after it is not copied.

Now the mysql_alter_table function forces table data copying by setting
the need_copy_table flag when it finds a moved column. The flag is set at
the stage when the modified fields are created.