When a ChangeField mutation changes both the field type and

unique/db_index attributes, MySQL's MODIFY COLUMN runs before the

index is dropped. MySQL rejects this for TEXT or BLOB columns with

the error "BLOB/TEXT column used in key specification without a key

length".
This happened because ChangeField.mutate() returns early after

change_column_type() when change_column_type_sets_attrs is True

(MySQL), skipping the change_column_attrs() call that handles index

drops.
change_column_type() now processes db_index/unique changes from

new_attrs when change_column_type_sets_attrs is True, emitting

index drops as pre-SQL (before the MODIFY) and index adds as post-SQL

(after). The SQLite3 backend also calls the new helper to keep in-memory

database state in sync.

Ran unit tests without the fix and verified that the new tests failed with

  the same issue I saw in the wild.
Ran unit tests with the fix and verified that it was now fixed.
Ran dbtests for mysql, sqlite, and postgresql.