The change in behavior was introduced by this changeset:

  ------------------------------------------------------------
  revno: 3405
  revision-id: joro@sun.com-20100317141846-es0qyf5zcqb0hu1c
  parent: davi.arnaut@sun.com-20100309125156-z2c4uyqque49v61k
  committer: Georgi Kodinov <joro@sun.com>
  branch nick: B49838-5.1-bugteam
  timestamp: Wed 2010-03-17 16:18:46 +0200
  message:
    Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may corrupt
       definition at engine
    
    If a single ALTER TABLE contains both DROP INDEX and ADD INDEX using 
    the same index name (a.k.a. index modification) we need to disable 
    in-place alter table because we can't ask the storage engine to have 
    two copies of the index with the same name even temporarily (if we 
    first do the ADD INDEX and then DROP INDEX) and we can't modify 
    indexes that are needed by e.g. foreign keys if we first do 
    DROP INDEX and then ADD INDEX.
    Fixed the problem by disabling in-place ALTER TABLE for these cases.
  modified:
    mysql-test/r/innodb_mysql.result sp1f-innodb_mysql.result-20060426055153-bychbbfnqtvmvrwccwhn24i6yi46uqjv
    mysql-test/t/innodb_mysql.test sp1f-innodb_mysql.test-20060816102624-6ymo37d3nyhvbqyzqn5ohsfuydwo426k
    sql/sql_table.cc               sp1f-sql_table.cc-19700101030959-tzdkvgigezpuaxnldqh3fx2h7h2ggslu