Trigger whole derived collection update when another observable changes

[HoneyTauOverTwo]

How can I trigger the whole MaskSourcesRelativePaths collection to update whenever MaskSourcesRootPath changes?

I guess WhenAny must be used to observe changes in both properties on the same expression? I searched for how to do something similar but was unable to find an answer.
 

public MaskSourceCollectionModel()
{
    this.WhenAnyValue(x => x.MaskSourcesRootPath).Subscribe
    (
        rootPath =>
        {
            // probably not the way to go
            UpdateRelativePaths();
        }
    );

    // any elegant way to trigger this for every item whenever MaskSourcesRootPath changes?
    MaskSourcesAbsolutePaths.ToObservableChangeSet()
        .Transform(absoluteFilePath => 
        {
            return MaskSourcesRootPath == null ? absoluteFilePath : FileUtils.MakeRelativePath(MaskSourcesRootPath, absoluteFilePath);
        })
        .ObserveOn(RxApp.MainThreadScheduler)
        .Bind(out maskSourcesRelativePaths)
        .Subscribe();
}

private void UpdateRelativePaths()
{
    // probably cannot go this route since it is read only
    //for (int i = 0; i < maskSourcesRelativePaths.Count; i++)
    //{
    //}
    // inelegant and inefficient
    for (int i = 0; i < MaskSourcesAbsolutePaths.Count; i++)
    {
        // triggers the change twice, unfortunately
        string original = MaskSourcesAbsolutePaths[i];
        MaskSourcesAbsolutePaths[i] = $"{MaskSourcesAbsolutePaths[i]} ";
        MaskSourcesAbsolutePaths[i] = original;
    }
}

[Reactive]
public string? MaskSourcesRootPath { get; set; } = null;

[Reactive]
public ObservableCollection<string> MaskSourcesAbsolutePaths { get; } = new();

private readonly ReadOnlyObservableCollection<string> maskSourcesRelativePaths;
public ReadOnlyObservableCollection<string> MaskSourcesRelativePaths => maskSourcesRelativePaths;