MySQL: change column type using expression
The correct syntax would be 

ALTER TABLE my_table1 MODIFY COLUMN my_uuid_column VARCHAR(36) GENERATED ALWAYS AS (1 +1)  STORED;


But you can't add many functions to a generated column, for example uuid() doesn't work, so you must do this in your code. where you generate the insert


Stored functions and user-defined functions are not permitted.
Stored procedure and function parameters are not permitted.
Variables (system variables, user-defined variables, and stored program local variables) are not permitted.
Subqueries are not permitted. 


See official Documentation