It's impossible to convert a table to some character set, but at the same time set the DEFAULT character set to another character set:

ALTER TABLE t1 CONVERT TO CHARACTER SET utf8mb3, DEFAULT CHARACTER SET utf8mb4;

ERROR 1302 (HY000): Conflicting declarations: 'CHARACTER SET utf8mb3' and 'CHARACTER SET utf8mb4'

The problem happens because the code handling both clauses uses HA_CREATE_INFO::default_table_charset to check conflicting declarations.

The test for conflicts was added by MDEV-7387. Before MDEV-7387 it worked even worse:

ALTER TABLE t1 CHARACTER SET utf8, CONVERT TO CHARACTER SET latin1;

ignored the utf8 part and just use the latin1 part for both conversion and DEFAULT.

It should be fixed to allow any combinations of cs1/cl1 and cs2/cs2 in:

• [DEFAULT] CHARACTER SET cs1 [COLLATE cl1], and
• CONVERT TO CHARACTER SET cs2 [COLLATE cs2]