Steps to reproduce:     1. ...   Result: ... Expected: ...   System info     Application: After Effects (Beta) v26.3.0.75     OS: Windows v10.0.19045, RAM: 127.70 GB GB, CPUs (logical): 28

Steps to reproduce:
    1. ...
 
Result: ...
Expected: ...
 
System info
    Application: After Effects (Beta) v26.3.0.75
    OS: Windows v10.0.19045, RAM: 127.70 GB GB, CPUs (logical): 28

I use extendScript to code relinking 3d models (glb).. This works in 2026 but not in the beta.  In After Effects Beta, FootageItem.replace(File) fails for GLB/3D Model footage with the warning: “Cannot replace a 3D model with this file.”  The same GLB can be imported by script with app.project.importFile(), and the same GLB can be replaced manually using Replace Footage > File. The issue appears specific to scripted FootageItem.replace(File) for 3D Model footage.  I am wondering if it is a bug or you are no longer supporting it.  You can run this test script to see what I am talking about.  
(function testReplaceOnlineGLB() {
var item = app.project.activeItem;

if (!item || !(item instanceof FootageItem)) {
alert("Select one online GLB FootageItem in the Project panel first.");
return;
}

var f = File.openDialog("Choose replacement GLB", "*.glb");
if (!f) return;

$.writeln("Selected item: " + item.name);
$.writeln("Before file: " + item.file.fsName);
$.writeln("Replacement file: " + f.fsName);
$.writeln("Replacement exists: " + f.exists);

item.replace(f);

$.writeln("After file: " + item.file.fsName);
})();