Cause sqlpreview event is triggert with all arguments by value these arguments will not change automatically and you can't change it. It is a real limitation.

So you have to handle it youself.

It would be easy if you could first change the argument in descendend before you change it in anchestor. In this case you can override the ancestor script, do your changes and call ancestor script with the changed sqlsyntax as argument.

But this will not work if you want to first change the SQL in ancestor because you will not have a way to get the modified sqlsyntax n your descendend. In this case you can use an instance variable to store the modified sql.

Maybe a better approach:

Create a new event mysqlpreview  (for example) that have a sqlsyntax argument by ref. Call this from sqlsyntax event in ancestor script. Do all your sql modifications in ancestor and descended in this new event.