CREATE OR REPLACE TABLE t1(a INT PRIMARY KEY, data TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin) ENGINE=InnoDB;

--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON

ALTER TABLE t1 MODIFY data LONGTEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin, ALGORITHM=INSTANT;

InnoDB does not distinguish between VARCHAR, TINYTEXT, TEXT, MEDIUMTEXT, LONGTEXT internally.
As long as the maximum length does not become smaller, the conversion can be done on the metadata level, without table rebuild (and without even validating the contents of the table).