Converting a composite primary key to composite unique, and adding an auto-incremented PK column
That smells a lot like a "many-to-many" relationship table.  Is it?

As for linking from a changelog -- space is not the issue; correctness is.  A change log should have exactly what existed at the time the log was taken.  What if the original table is updated?  If the table in question is changed for any reason, I think you do not want the change reflected in the existing changelog entry.  (Though, perhaps in a new changelog entry.)

If you would like to discuss your particular case further, please provide SHOW CREATE TABLE and a few queries that use the table.

Here's another tip: It is OK to do the following:

id INT UNSIGNED AUTO_INCREMENT,
...
PRIMARY KEY (...),
INDEX(id)


That keeps the original PK clustered, which might be beneficial, yet still allows for an AI key.

Another tip:  Don't say UNIQUE(foo) in place of INDEX(foo) unless there is some reason for doing the uniqueness constraint.  (Example:  INSERT ... ON DUPLICATE KEY ...)

Further discussion about many-to-many.