alter_table.result, alter_table.test, field_conv.cc:

  Bug #6479  ALTER TABLE ... changing charset fails for TEXT columns
  Fix: use do_conv_blob rather than do_copy_blob
  if the column's character sets are different.

mysql-test/r/alter_table.result

@@ -483,3 +483,13 @@ ERROR 42000: Incorrect table name 't1\\'
 rename table t1 to `t1\\`;
 ERROR 42000: Incorrect table name 't1\\'
 drop table t1;
+create table t1 (a text) character set koi8r;
+insert into t1 values (_koi8r'');
+select hex(a) from t1;
+hex(a)
+D4C5D3D4
+alter table t1 convert to character set cp1251;
+select hex(a) from t1;
+hex(a)
+F2E5F1F2
+drop table t1;

mysql-test/t/alter_table.test

@@ -324,3 +324,15 @@ alter table t1 rename to `t1\\`;
 rename table t1 to `t1\\`;
 drop table t1;
 
+#
+# Bug #6479  ALTER TABLE ... changing charset fails for TEXT columns
+#
+# The column's character set was changed but the actual data was not
+# modified. In other words, the values were reinterpreted
+# as UTF8 instead of being converted.
+create table t1 (a text) character set koi8r;
+insert into t1 values (_koi8r'');
+select hex(a) from t1;
+alter table t1 convert to character set cp1251;
+select hex(a) from t1;
+drop table t1;

sql/field_conv.cc

@@ -475,6 +475,8 @@ void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
   {
     if (!(from->flags & BLOB_FLAG))
       return do_conv_blob;
+    if (from->charset() != to->charset())
+      return do_conv_blob;
     if (from_length != to_length ||
 	to->table->db_low_byte_first != from->table->db_low_byte_first)
     {