Is it possible to override property types when extending a component in CF 2023?

I can confirm what you have found. It happens not only in ColdFusion 2023, but also in ColdFusion 2025. It is understandable that you expect the CF 11 code to work on CF 2023. After all, new ColdFusion versions have a tradition of promising backward-compatibility. However, I think that that is an unrealistic expectation in this case.

The gap between CF 11 and CF 2023 is 4 major versions. The time span is 9 years. Besides, CF 11, CF 2016 and CF 2018 have all passed their end-of-live. So, in all likelihood, Adobe won't pay close attention to these old versions when modernizing ColdFusion. 

With each upgrade to a new version, the Adobe team does a lot of pruning and optimalization. As a result, recent versions of the ColdFusion application server are less weakly-typed, more accurate and more closely aligned with Java. Also, each new ColdFusion version deprecates some of the outdated software. 

That said, the example you give here involves a fundamental functionality of ColdFusion, of the underlying Java or of any other object-oriented language for that matter. Namely, the inheritance of the fields of a superclass by a subclass. In other words, access by a subclass to a field of the superclass.

With regard to this specific topic, I am not aware of any changes between CF 11 and CF 2023. What ColdFusion might have made stricter since CF 11 is the way child components inherit fields from parent components. I would expect ColdFusion to do this in line with Java. 

In Java you ensure this in one of two ways:

1.  declaring the field as "protected" in the superclass;
2.  letting the subclass declare a variable with the same name as the one in the parent class. The variable of the parent class is then said to be "hidden" by the child class, not overridden. As far as I know, you can only override methods, not variables.

Your example is of the second category. So let's look closer into case 2.

Suppose the "hidden" field is of reference type. For example, a query, as in your case, Then, in Java, the class (type) of the field must be known at compile-time. Whereas, method-overriding and field-hiding happen at runtime.

I would expect the behaviour to be the same in ColdFusion. That is, the type of the MyData property is locked in as Query at compile-time. Hence the result that you got. 

A possible workaround is to use method overriding. This, you will recall, may happen at runtime. 

The method to override is, of course, setMyData:

/* Extend.cfc */
component accessors="true" extends="Base" {
    public Extend function init() {   
    	variables.setMyData(queryNew(""))	
        return THIS;
    }
    public void function setMyData(q) {   
    	variables.mydata = ["123abc"];
    }
}

/* testPage.cfm */
extendObject=new Extend();
writedump(extendObject.getMyData());