Impossible to import a lower version of Quickcontrols when build with higher version of Qt

the background is that, My app is written with Qt5.13, the Combobox does not have property "currentValue" in this version, So I defined one. 

But When I upgrade to Qt5.15, this property is added, So the app will crash with error"Cannot override FINAL property".

I don't want to modify my source code because the property "currentValue" is used in many places. I tried to write like this. but failed. 

// code placeholder
import QtQuick.Controls 2.13 as QC13
QC13.ComboBox {
property var currentValue: ""
}

 The app will still crash , the error is same. 

It seems that, If I use Qt5.15 to build my app, I can not force to use controls of 5.13 , right?