"Alter Table" modify multiple columns vs multiple Alter table single column in INNODB?
You must do it all in a single ALTER TABLE command. Think of it : You are changing the collation of a column, so ALGORITHM=COPY must be done. This requires a full table lock. It is better to execute the collation change of multiple columns in a single ALTER TABLE command than to do it multiple time
There is no drawback in this instance. Why ? Back in May 12, 2011, I wrote about MySQL 5.0's old behavior in executing multiple column modifications (Does MySQL still handle indexes in this way?). Under the hood, MySQL used to execute multiple column changes as individual ALTER TABLE commands. I pointed this out to MySQL back on Oct 10, 2006. They had fixed it since.
Now that ALTER TABLE handles multiple column changes as a single operation, there is no need to change the columns one by one.
Let's say it takes 5 min to change one column. That's 5 minutes the table is locked and no INSERTs, UPDATEs, or DELETEs can happen. You would basically double the time changing two columns to 10 min, triple the time changing three columns to 15 min, and so forth.
Just run the one ALTER TABLE command. MySQL handles it correctly now.
ALTER TABLE mydb.`cwd_group`
    MODIFY `active` char(25) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL
   ,MODIFY `local`  char(25) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL
;

GIVE IT A TRY !!!
P.S. You need to specify a length for char. That's why I put (25).
If you are not sure of the length, then use VARCHAR(25).